This is a very fun and intimate act that I've been lucky enough to do a number of times. The first time I tried it after I had come in her ass and boy, did it take a while for me to mentally relax and be able to let go. I really needed to concentrate on relaxing and release all the 'rules' my body has been taught about where to pee! Luckily it gets easier and easier each time.
With one partner on a number of cold wintery mornings, we would fool around which always led to anal sex. With her I didnt have to ask and I'd just stay inside her and relax to let my morning pee into her ass. She'd eventually feel it and laugh and ask if I was pissing in her. I told her why should I have to get out of a cozy warm bed to do my morning piss, when she could do it for BOTH of us! LOL
But in reality I got up with her and while she was pissing out of both holes, I prepared a small enema for her. I just thought it was a good idea to give her a rinse after she expelled my urine.
Very fun times that I hope to repeat...