I have tried, but it was not so easy. I think it is right to mention - I knew my partner very well. We had been lovers for years, trusted each other. Both free of disease etc. This is not something you do on first date!
Well - I lubricated her with KY Jelly and entered her rear. Then I started to concentrate on peeing. Not easy! I started to shrink, managed to get a few drops in her before I slipped out. It was fun, but not very successful.
Would I do it again? Sure! My partner loved it (She loves about everything humiliating) We talk about doing it outside next time. On a hike when both of us need to go I will empty myself inside her and when I am finished she will squat down and both pee and empty her rear