Legends of the Five Rings does offer rules for cooperationL5R 4E Core p.81, mainly they seem aimed at mundane tasks like building a ship or designing a new garden.
However, some spells, like Bonds of Ningen-Do (a ritualistic spell taking 10 minutes, shortened by one per assistant)L5R 4E Core p.177 have specific rules that there are assistants. This seems to hint that spellcasting can be assisted at.
Assuming two Shugenja have learned the same spell (or are instructed for a rite such as the one above) and they have access to all their needed materials, the rules on p.81 offer 2 variants on how to handle cooperation rolls. Interpreted on spellcasting that gets us:
- A lead-caster is chosen and he gets a bonus of cumulated school-ranks (as they replace skill-ranks) on the roll as a static value.
- All roll but the caster with the highest school rank does grant a bonus of their bonus to any assistance.
I am highly leaning towards 1 in most cases, as the worst that can happen is wasting a spell slot, but a group of combat casters might also work together under the guidance of a master shugenja, each casting their own spell simultaneously, which sounds like a type 2 case.
Yet some complications have to be kept in mind with finding a good ruling:
It's unclear if variant 1 means that only the main caster expends a spell slot or do all assistants need to spend either a matching spell slot or void spell slot. To me, it sounds like a huge tax on the helping shugenja, for offering a few fixed points. That valuable slot might be more effectively used by casting the same spell twice, so I would not impose the spell slot cost but they need to spend their actions on the casting just as much as the main caster.
Variant 2, as 'group casting' isn't without any problems either: There are usually little consequences to someone messing up their spell but the wasted spell slot. So technically the whole variant should not be an appropriate choice to assist one another in the first place. That could be remedied by ruling that a single failure in such a caster group also fails all of the other spells: either you get a full barrage of the same spells or none at all. That would make such a caster group a heavyweight on the battlefield, matching some in-universe stories where a group of few (45) Phoenix shugenja defeated a whole army at Kawa Mitsu Kishi (Three Stone River).
Do I overlook a problem with these two solutions and do they offer a balanced approach to assisting one another in casting spells?