Potion Merger is a primary ability.
You can merge two potions by holding them in both hands and pressing the ability keybind.
You can merge any effects, and any amount of effects, into one potion, except you can't merge two of the same effect.
Examples:
Strenght 2 + strenght 2 doesn't work.
Strenght 2 + strenght 1 long does work, and gives you a potion with both effects, which works by giving you strenght 2 for 1:30, and strenght 1 for the remaining time (6:30).
You can now also add any other effects to this, for example regeneration 1 long and regeneration 2.
You can only use these if you have the ability equipped.
Obtaining
You get this ability with the Furious Cocktail advancement