Bard’s song buffs can’t stack with other players songs buffs.
So like if you have three Bards, one uses Ballad, one uses Paeon and one uses Minuet, I think the party list will display Minuet as being on your party members since it was the most recently played.
The buff icon will also quickly move between Ballad / Paeon / Minuet since from what I understand they’re technically overwriting each other every few seconds, but beyond that I’m not entirely sure which of the buffs is actually being applied in this instance, so hopefully someone else can say more about that.
I’d guess that since you’re getting three different buffs that are only applying for seconds at a time, it’d maybe be less effective than the standard one buff every 30 seconds. But then, the songs are so crucial to Bard that you can’t exactly opt out of using them. In the instance that both Bards are singing the same song at the same time, I imagine it’d give the same effect as if just one of them were singing (assuming they both started/stopped at the same time)
Basically, you can’t have more than one song effect on the party even if you have more than one Bard. So that maybe means it’s best to sync songs with another Bard in raids etc so that there isn’t any chance of overwriting one buff with another.
That’s just how I understand it though, so I may be wrong, especially when it comes to overwriting