Clever alchemist used to let us have 2 potions active at the same time, now it's harder to do. Afaik you can have potion glyphs on your jewellery and potentially have 2 potions active at the same time but nowhere near as effective as clever alchemist used to be.
Just checked online this is wrong, there is an old build I was thinking of.
Still, with 3 infused jewellery and potion cooldown glyphs you can take a potion every 21 seconds. Essentially you can have 2 sets of potion buffs active at one time.
Once you use a potion, there's a cooldown. So you can only have one active at a time.
This.
However, with the passives from alchemy to extend the lingering effects, the longer effects do last a couple of seconds longer than the cooldown for using potions. If you use a potion with different lingering effects than the first potion you can have additional "buffs" for that couple seconds if you use the potion immediately when it comes off CD.
using the same potions will just refresh the same buffs, they will not stack.