Ive looked everywhere for the 40% enchanting potion (can't remember the name) and I can't find one. but Ive found a 15 and 20% one. if I drink both of them, will it boost my enchanting up to 35 or will one replace the other one?

It will not boost enchanting up to 35. If you drink both, it will be 20%.

There is no 40% enchanting potion in game. 25% is best you can find, Enchanter's Elixir.

I thought there was a 40%. But tyvm!

You can make one that does 2000% if you use the Alchemy loop. 100% boost makes you immortal anyway.