This is something I wanted to do. Apparently its a good combo for some people, thats because Lex has a that famous drowsy, sleepy, apathetic effect whereas Wellbutrin has the energetic, super highs (but supers lows I believe) etc effect. So they balance each other out rather nicely. But this is not from my own experience since I'm only on Lex. Hope that helps you a little bit.
Take car Pierre.