I tried to locate this topic in this forum but failed. How do you treat combinations of forms. For example if you wish that a breeze is warm then you just add a Ignem requisite to your Creo Auram spell, no worries there but if you would add a heat that could hurt someone fast in a strong wind of let’s say storm strength. How would you deal with that? Two different spells, one for creating the wind and one for the heat or any other suggestions. And yes for those who wonder I have some plans to use suggestions here in the Aelianus works in the Novus Mane story.
Use the Form base level that is the higher one.
Or set it up as 2 different but lower level spells cast in succession or at the same time if the magi is capable of it.
I like spells to do one thing and one thing only, thus if the effect can be broken up I believe that it should be.
There is of course the +1 magnitude level adjustment for a non-essential requisite, yet I believe that this is for an increase in the applicability such as a spell that make a hole in a stone wall also applicable to wooden walls rather than increased functionality of the spell like a spell that both paralyzes a target (ReCo) and removes its metal weapons (ReTe).
I don't like the second combined spell at all.
If your distaste is less intense than my own, (and why shouldn't it be), you might consider a level for the combined spell of:
Level of highest spell + the level of the second spell or three magnitudes, whichever is greater
I say use the highest base level, then add a magnitude for an enhancing requisite. If the effect is exceptionally fancy, add two. I myself do not like limiting spells to just one function. A well designed spell could do it all in one shot, just make the level higher. For example, a CrReCo spell that conjures a corpse and animates it in one spell (conjure the skeletal warrior).
As others have said - use the highest base.
If the additional effect increases the usefulness of the spell add a magitude. If it differs greatly from the original spell add at least 1 more. And if it increases the usefulness by a lot, add some more even...
I.e.
A spell that makes you invisible, heals a light woudn and teleports you home might be ReCo(CrPeIm). The teleport is the most powerful effect so base it on that. 3 requisites that add something (+3 mag), Two additional very different effect (+2 mag) - but they don't really add that much additional usefulnes so we'll leave it at that...