I asked about this on the Discord but only got replies from two folks, and both seemed like "the rules don't say, so this is what I think..."
If two different magi cast a spell on an object, is there any iteraction between the spells?
if the spell affects the same property of the object (say, color)
Does the second spell cast have to overcome the first spell?
If the second spell is shorter duration, does it "turn off" the first spell until it expires, at which time the second spell reactivates?
To think of an example, say that the first magus casts a spell to turn a staff white for Sun duration, and the second magus turns the staff black for Diameter duration.
If the spell affects different properties of the object, how do the spells interact, if at all?
As an example, say the first spell turns the staff to stone, while the second spell makes the staff blue.
Let's talk Rego. If the two magi are trying to telekinetically wrest control of the staff from each other, let's say with ReHe spont magic. Assume Concentration duration. How do you decide which magus wins and pulls the staff to him?
For the first two cases, the spells won't really interact at all.
If spell 1 turns an object red, and spell 2 turns an object blue, then the object will first turn red and then turn blue as each spell takes effect. If spell 2 ends before spell 1, the object will no longer be blue, but be red until spell 1 ends.
For your second example, you will end up with a blue stone staff.
There will also be a bunch of edge cases, depending on what kind of spells and properties we are dealing with, and how they change a property.
If an object is turned red by a Muto spell, it will simply revert to its old colour when the spell ends.
If you instead use Rego Craft magic to turn an object red, you have basically painted it red, and the object will remain red when the spell ends.
For your third question, I don't think there any rules covering that case, so you will have to make some house rule.
Highest spell level wins, or highest penetration wins, both sound reasonable, but I am sure there are plenty of other options.
For two competing Rego TK spells of similar parameters, I would tend to rule in favor of a competing Finesse check which is frequently used in Rego for fine control and seems appropriate here, whoever wins pulls the staff in his direction. This might, situationally speaking, stretch over more than one round.
I have to agree with Timothy here, the color changing might be as well if it is concentration duration. If you cast and forget then the latest spell will take precedence.
I would think the staff-pulling would depend on a few things.
If someone uses a stronger spell (not necessarily higher level, but literally stronger), shouldn't the stronger one win? For instance, with Unseen Arm v. The Unseen Porter, Unseen Arm can't fight much resistance, while The Unseen Porter can pull at Str +5. I really can't see Unseen Arm ever winning out against The Unseen Porter regardless of Finesse or anything else.
If there isn't such an inherent strength difference, only then would I turn to something else. Personally, I'd probably default to competing Finesse next, but I could see other options.
The question was with regards to a ReHe spont spell, which is why I would go with Certamen, which includes strength in the arts involved as part of the competition. For formulaic spells I would use the stronger spell criteria. Finesse tends to focus on fine control, I think for a raw test of "strength" where all else is balanced I would either simply use a straight unmodified stress die competition or else make it a contest of penetration.
I get that it was about sponted spells. Sponted spells are the same as formulaic spells; they just have different casting totals and possible Fatigue losses. (Yes, for counter-magic we have a quick-and-dirty method to speed things up, but that's just a time-saving measure for the troupe.) Consider these two (using Terram just to use the same two core spells, but it works the same with Herbam):
Magus A spontaneously casts Unseen Arm. Magus B spends 1 Fatigue to spontaneously cast The Unseen Porter. If we go with Certamen, Magus B is extremely likely to lose. Why is Magus B's The Unseen Porter (Str +5) so much weaker than Magus A's Unseen Arm (cannot overcome significant pulling)???
Because as they realize they are in a competition with each other they begin manipulating not just the rod but the magic they are using to manipulate the rod. The one who actually knows the arts instead of being specialized to get a bit more oomph will be at an advantage.
Admittedly I might allow the virtues and flaws to apply since this is a contest within those specific domains rather than being general and formal certamen, but that would most definitely be a storyteller call, not something you would want to devote pages of rules to
So, from what you've said, if you use Unseen Arm and The Unseen Porter formulaically, The Unseen Porter wins. But if the same spells are spontaneous, then the can manipulate the spell more easily after having cast it than if it were formulaic, so then it goes to Certamen. But why can you manipulate the spell more easily afterward if it's spontaneous? Or go the other way, why is it that a caster of a formulaic spell cannot realize they're in a competition? And if you're using two different sets of rules here, what happens when a formulaic one goes up against a spontaneous one?
If you are using a formulaic spell you are following a well defined and pre-determined recipe, which allows you to cast at a higher level due to the time you spent developing the formula. As such it is less flexible than someone who is winging it but not able to generate as much power as if they had put in the time to develop the proper recipe or technique. Consider if you will someone who did realize after casting a formulaic spell that they were in a competition and so they cast a spontaneous version to give themselves that flexibility- the immediate drop in the power of what they can cast is answer enough.
No, I didn't ignore it. I took that into account in my statement. Even your second paragraph makes it better for the weak sponter. Even if you double an Art for the good sponter, unless you also include some house rules about the lack of spending Fatigue on sponts to alter the Certamen and also either refund the spent Fatigue from the better caster or artificially make the other one spend Fatigue, there are multiple ways the better sponter lags behind in the Certamen contest.
Not certain what you want, there is no way a single virtue or flaw should just hand the contest to the other person. You contrived a specific example so that one person would have weaker arts and a more powerful spell in that instance. A munchkin is a munchkin, and when you min-max you have to expect downsides.
I chose that example to make the issue blatantly obvious. All I needed to use were two identical magi, one who spends a Fatigue level to spont the stronger spell, the other who doesn't and sponts the weaker spell. Why is the weaker spell stronger than the stronger one. That's not of a "munchkin" case, is it?
What am I asking for? Something internally consistent so that even the person proposing it doesn't give misunderstand its results? Something internally consistent so it can be used between spontaneous and formulaic magic? Something consistent with regard to the rules for ArM5 without adding house rules?
Any answer to this question will involve house rules, so what you claim to be looking for you cannot find.
It should be noted that certamen already involves the loss of fatigue levels for the loser, as well as adding stress dice. If the other magus is not willing to engage with those rules they are effectively bowing out of the competition, so the magus willing to spend fatigue would automatically win.
What I meant by "adding house rules" is adding house rules on top of whatever is chosen for this. Sorry, I wasn't clear.
You're either confused about certamen or confused about the math. With your method, by not spending Fatigue on the spell, you save yourself 1 Fatigue to use in the certamen and potentially start with fewer penalties. Thus, with two identical magi, the one who choses not to spend the Fatigue has the odds favor them as compared to the one who spent the fatigue to cast the stronger spell.
I don't think it would be a stretch to simply count that one fatigue against the first loss of fatigue to certamen. That is at best a minor quibble based on a very specific case and how accounting of the encounter might be handled.
If person A casts a spontaneous spell and person b casts a stronger spell to try and counter it using fatigue they are clearly attempting to engage in conflict with the original spell. If this is resolved with certamen using the arts involved then the character would have to be completely ignorant that this is how things are done in order to make that decision. As such what you are talking about is someone making a move that would inherently be a lousy choice given what is the best resolution to the conflict.
But lets say for the sake of argument that we do things you way. Person A casts a spontaneous spell to move a tree limb. Person B uses fatigue to cast a stronger one to combat them. Then Person A can simply cast a new spont spell using fatigue to overcome the Person B's spell. The fundamental concept of conflict using fatigue to overcome your opponent is preserved whether you use certamen or not, and your hypothetical mage B who would only have temporarily won due to the use of fatigue still loses. I simply feel that certamen makes more sense in game terms.
Although I don't like adding complexity to the existing body of rule, I like the idea to use Certamen for something else than just Certamen duel. Maybe in this case, each mage has a bonus equal to the magnitude of the spell they used to counter each other ? It would justify why use a higher level spell.
Sure, if one mage is clearly more powerful than the other (like in the case mentioned above), the magnitude difference will be of little importance, but at least, it differentiates both spells and gives a little edge in case of similar power mages.