My wife knows that my asshole is really sensitive. Sometimes when she is giving me a handjob and it isn't working (she doesn't like to give BJ's) she will finger my asshole and that almost always does the trick. Her's is sensitive as well, but she is pretty squeemish about her asshole being pentitrated, so I just have to take it easy. One time I was going down on her with my tongue, and inserted a finger into both her vagina and ass and after a short bit of time she said "too much.. too many snesations, I can't take it". I stopped, but grinned to no end.
