If I drink 2 blacksmithing potions will it double the the effect? I have 2 potions that will improve my smithing by 20% for some time. If I drink them both, will it become 40%?
Potions do NOT stack. So no, if you drink them both you won't reach 40% in smithing.
The only potions that stack are the restorative ones.
Last I played Skyrim potions which were stacked in your inventory/have the same strength would not stack, so in your case no they will not.
However if you had two potions of different strengths, say 20% and 15% they would stack multiplicatively ie 1.20x1.15 = 1.38 so those two together would give you 38%. It is very likely that they patched this at some point though because you could make your own potions and have many different strengths of potions and then stack them all for very large bonuses.