All I can find is about one maga casting a spell. Are there options for several magi casting a spell together? I thought it was possible for rituals with older editions, but I didn't manage to find any text about if this is still possible or how it works.
Aegis of the heart used to be a prime example of several magi working together to create the most powerful aegis they could manage. What is the new status? The most powerful specialist handled the aegis, with everyone else attending the ritual without actually contributing? Or is a specialised version of the Wizard's Communion now part of the spells most magi learn and every covenenat keeps in its library to allow other magi to chime in and boost the ritual?
They just use a longer-duration version, usually D: Sun. The D: Momentary (up to 1 round) one is useful for non-ceremonial formulaic magic (unless the one casting the spell has the Slow Caster Flaw). The effect of the D: Sun version is the same as a D: Momentary version 2 magnitudes lower.
In Mythic Locations, p96, a lost (Duration= Concentration/Special) Mercurian ritual interact weirdly with Wizards Communion
It makes no comment on what duration the wizard's communion must be, and explicity calls out that you can't carry these rules over to other spells, but what would the result off integrating this effect?
Though I don't have Mythic Locations, it looks like that ritual had D: Concentration. It's special design is that all of the magi are considered the caster of the spell, so any of them can provide the concentration to maintain it. If this were done more, there would be more rituals of D: Concentration. However, each time this is done the total number of magi who can do it drops by one. But since we can't just leave a single magus concentrating forever, really it's more like three magi are required to keep this going forever instead of one. Let's say in each tribunal there will be one such ritual maintained forever. That would require 3% of the Order's Gifted population to be forever tied to maintaining the rituals and really never doing anything else. Then, if it's a season of service a year - a far more likely regular sacrifice than four seasons every year - we need to quadruple that. 12% of the magi in the Order are required, though now they have some of their own pursuits. But now it's looking like something that will only be managed in the Transylvanian Tribunal or in a particular instance. So I doubt integrating this concentration effect will have a broad-reaching impact due to practical reasons.