i think the new organ creates itself purely for reproductional purposes, at least in here.
If not tho id guess they pee from their original urethra as their body is used to that or it goes all weird and pee would come out of both at the same time uncontrollably.
The option to choose wouldn't make sense because those are muscles that are foreign to them. With time and work with them id say they'd be able to stop the pee from coming out of one just as we can control it to some degree but itd still be pretty hard as they'd work similarly and be so close together control would get a little messy.
thank you for indulging on a silly (of course unserious) question i had! i totally forgot they grew new organs (duh)
broaching the age old debate… is squirt pee?
NOW WE'RE COOKING FOLKS
does having two urethras imply having two bladders? which do they pee out of? both? can they choose at will?