Originally Posted by RivianWitch
Do potion effects stack if you have him drink 2 of the same potions at the same time? Say 2 Swallows at once, is that = Swallow effects x2?

Nope as people said. You can naturally quaff a Gadwall and Swallow if you like although
White Raffard's decoction + Gadwall should be enough of a buff for about any fight (Just
make sure you invest on catalysis and potion Specialization if you really want them to
make a difference in this game imo).