Me and my best bud have been doing this for a couple years now. We jack each other off a lot but one night we were really horny but didn't wanna go all the way because we thought it was a bit too much.
Basically one of us lays on our back with a boner and the other sits on it so the cock runs up the ass crack. The one on top then rubs their ass up and down the other's body over and over. It feels AMAZING and I always cum the most when we do this.
Sometimes it can be even better when you lay them on their front and you kinda dry hump their ass. Its even better if you use something as lube like shower gel or KY Jelly.