Simultaneous were always frequent for me because when my partner began to orgasm, hers would trigger mine, causing my ejaculation. Her moans and groans coupled with her body spasms and pelvic gyrations just did it for me.
I've had a couple of "encounters," J/O, but fantasize about M2M sex daily, particularly oral, especially a 69 where we both ejaculate into each other's grunting mouths at the same time. However, fantasy is typically better than reality, but I'd certainly like to explore!
Well, it can be visually appealing; besides it's just another activity in a long list of sexual play.