Right so I actually went into the rabbit hole for this. Majority of the answers was basically as long as the top doesn't have an infection, its generally okay? The end result would probably be like an enema. But because the colon is very absorbent you still kinda absorb some of that piss essentially? Which actually might not be good, but people who says they have tried it, say long term there wasn't any health issues so I guess its fine (not a doctor)
TMI I've tried it in both holes and I don't get the appeal of it. Like you'll feel it (the pee) but it's just like if you use a douche or an enema, and it's mess to clean up so I recommend doing it in the shower than the bed. Thankfully, I didn't get any infections because my fwb is a hypochondriac when it comes to his kinks lol.

if i had a nickel for every manhwa where the bottom is pissed in i'd have 2 nickels. don't get me wrong i'm a water sports supporter but piss in the asshole cannot be healthy.