I am interested in people's interpretation of the Performance Magic rules listed in The Mysteries Revised Edition in regards to casting additional spells.
As I read it, the Duration: Performance allows you to continue a spell without having to maintain concentration. You just have to continue performing the underlying skill. I do not see anything that indicates whether you can cast additional Duration: Performance spells and maintain them all together, or if you must end an existing performance/spell in order to start/cast a new one.
My interpretation would be to allow multiple spells to be maintained with a given performance, but still require a Concentration roll for the actual casting of the second (and third, etc.) spell, just not require them for maintaining the spells.
I wouldn't require a concentration check: Performance magic already requires an ability check when casting a spell. If you tried to cast a second spell and failed the roll, I would terminate your original spell too. I probably wouldn't make the second spell harder to cast than the first, but I'm not firm on that point.
I would require a Concentration check to cast another spell. I just wouldn't require one to maintain the current D:Performace spell. I'd rule that casting a spell and continuing to play your instrument was too confusing, requiring a Concentrating check.
This is mostly for 'realism' and 'balance' reasons - I just can't imagine casting a spell while playing out another as if it's easy, and I think Performace Magic is too powerful as it is.
Thing is that Performance magic doesn't require you to (assuming Sorcerous Music) play specific notes to maintain the spell. Just that you play. So if you want to cast another spell with Performance magic, you can just play whatever notes are required for the new spell (as the "Sorcerous Brawling" side-note indicates, you need a specific performance to cast the spell). The mere fact that you're still playing music is sufficient to satisfy the maintenance requirements of the existing spell.
Which is why I'm willing to increase the difficulty of the ability check, although for long-term activities like Hunting, it is difficult to see the casting of another spell as a significant interruption.
That's an interesting distinction I didn't consider before. Hmm. Considering the example of Weapon Ability use, this does seem to be the case.
It seems that indeed a Concentration roll is therefore not really needed. I would, like you, still recommend increasing the EF.
But it isn't pretty, I think that should I SG a saga with Performance Magic in it I will employ it with some house rules. I would say that you need to maintain the same activity to maintain the magic, not just engage in any Ability-related activity. So you would need to maintain the same song to maintain the spell (think long bardic saga...). I would furthermore even require some thematic relation between the spell and the performance. Having the people wallow in sorrow (Weight of a Thousand Hells) as long as you keep telling them jokes (Performance Magic: Storytelling) just doesn't feel right.
This would imply casting another spell might be impossible with performance magic while maintaining another. I'd agree to relent in the Concentration check, however I'd require a higher Ability check - you need to weave an even more complex melody, incorporating both spells. I'd require a +3 to the ease factor, more if I feel the new spell is somehow in opposition to the maintained ones. Probably cumulative too.
I would maintain that casting another spell normally, without employing Performance Magic, is still rather distracting from the performance and require a Concentration check to pull it off.
I'd be willing to allow someone employing Hunt to cast various hunting related spells without the +3 increase to EF, because the activity is so slow and broad and beause it's such a weak ability when compared to music. I'd still require him to persist in the same hunt - once he's moved on to a new hunt the spells would be broken.
Regarding a Weapon Ability, I would probably say you need to perform a 'kata' to awaken the spell as per the normal Performance Magic rules, and then need to continue to engage in battle - as long as you engage in the same battle, you are still engaging in the same 'activity'. I assume the positions, style, and focus adopted during the kata are maintained in the fighting, so the 'activity' remains. I would require an increase in EF as above for casting more spells, as combining fighting styles is difficult.
In short, I'll go over the specifics of how to interpret Performance Magic in-game and how it works with the player, on a case-by-case basis. Different Abilities require different interpretations and rules IMO. It's prettier this way, feels more right.