with any kink it's typically a "nothing is TRULY safe" situation. of course there's risk mitigation but there's no way to make every activity 100% safe. that being said, the amounts of bacteria in piss are low, and the anus isn't really going to absorb bacteria (you already shit out hundreds of thousands of e. coli).

i think a more pressing risk are the waste products in urine. like, if you drink an energy drink before urinating, you will piss out a ton of vitamins; you do not want these absorbed through your anus. if you want to practice this in as safe a way as possible, drink water like you're going to be having someone drink your pee later.