Is there a mod that let's you do this, like bind two keys to one function at once and two functions to one key? It would be pretty convenient to just equip my weapon with my left mouse, and it would definitely go well with the morrowind patch that allows you to instant cast.

I think the closest thing to that would be to use the MGE Macro editor or JoyToKey to map a quickkey and the use key to the same button.