My wife and I did this, she was very creative and found a lot of ways to do this. My first serious girlfriend recognized that I was very shy. She would have me masturbate for her from time to time. The best was when she talked me into masturbating for her and two of her friends. We began to realize that things that embarrassed me also turned me on.
My wife and I occassionally will sit facing each other and masterbate. It doesn't happen very often, but when it does, it is enjoyable.
I'm always so surprised reading how many people don't masturbate with every sexual partner they've had and feel like it's a special or kinky kind of thing on threads like this. I've always masturbated with every woman I've dated. Sometimes together. Sometimes just one of us watching the other. It's never taken a conversation or some kind of "Well... You know what I'd really like to try....?" And every female, male and genderqueer friend with whom I've discussed this is the same. I'm not saying this to be like "What is wrong with you people???" I'll admit I'm surprised how often I see people saying they want to but never have or that they do but describing it as a really far out thing their partner allows once in a while. But what really confuses me is how come everyone I know IRL regards it as common a sex act as hand jobs, blow jobs, fingering and cunnulingus, yet every mutual masturbation thread is overwhelmingly the opposite? I just can't think of a single reason why the responses would swing so heavily one way or the other based on the forum in which the discussion is being had.