My jo-buddy and I haven't done mutual masturbation, but jack ourselves off in front of each other. At first, we jacked off at the same time (that's when you pretend not to be looking at the other guy's cock, and just sneak looks). Pretty soon, though, its obvious you're watching the other guy's stroking, so now mostly we do it one at a time. Its actually better I think one at a time, because if you are the one doing the stroking, you can concentrate on your own cock, you speed of stroking, pressure, everything, and enjoy every last pump of your cum. And the other buddy can concentrate on watching every movement of your cock and balls, and see every spurt when you finish. Like I say, we haven't jacked each other off, but quite often I'll just hold my buddy's cock or sack in my hand when I'm stroking myself.