I'm nothing if not helpful

To the guy (And that's just a guess, but I think it's a good one) who landed here via a Google search for "shared peeing shower etiquette," first let me say that etiquette dictates that, in that situation, the thing to do would be to not pee in the shower. If you absolutely cannot hold it, I suggest explaining your situation to the person you happen to be sharing the shower with, excuse yourself briefly and use the toilet. If said bathroom has no toilet, and you can't hold it, the sink is of course an option, albeit a rather offensive one. And if there really is no other option during your shared showering experience than to relieve yourself right there, I suggest aiming away from the other person. If this is a situation you find yourself in often, however, I'd keep myself hydrated and avoid the asparagus.