Say you use a potion that gives you 20% protection from fire damage for 60 sec, then drink another one of the same kind. Would that mean you would have 40% fire protection for 60 sec?

Can I?

No, I don't think so.

Of course it would, because the game is supposed to be a cakewalk, right?

-Spams 10 of the cheapest potions in the world and becomes a god- YES!

Ehh.. no.

I believe you cannot, based on the fact that fortify enchanting potions do not stack for instance.

No, you can't.

No stacking of fortify potions, not even of potions of different strength, weakness poisons stack Oblivion style you can poison someone with first weakness to magic and then weakness to fire and they will be serious hurt by fire damage.

Think stacking smithing potions would have pretty devastating effect 148%+ 140%+ 130%.