Originally Posted by
Palistarius
The same way mirror classes grant the same buffs (Jedi Knights and Sith Warriors grant same stats boost), mirror companions will likely also follow suit. Same way Druids and Paladins have overlapping, non-stackable, buffs in WoW. Mark of the Wild and Blessing of Might, I believe. Could be Kings, little behind on Pala buffs tbh...
These legacy buffs cannot be stacked.
According to a swtor forum thread i found, only Prescense buffs are stackable. I'd link it, but I've not posted enough yet.
But I'll be honest, this is speculation as I don't know anyone with multiple Level 50s or max affection companions. Only have a 50BH and married to Mako (Endurance buff with Prescence bonus).