ok, found a recent video of party with Shingen https://www.youtube.com/watch?v=iUZplSCkJGQ Shingen's assist is increasing burst gauge rise amount, but here we can clearly see that other hime only get 10 burst points per hit while Shingen herself in first round gained 25 for double attack, so 12.5 per attack. Considering thats video from april this year, its safe to assume that devs dont give a fk that assist skills for Souls dont affect other party members.