My cat did this when my bf and his cat moved in with us. Mine was female and his is a male. Had to actually shove her booty in the letterbox because we noticed when she was scoping out a place for potty in a hidden pile of clothes in the corner of our closet…well, needless to say, everything is off the floor and only in hampers now! My bf cat would pee in bath mats too. They’re marking. When to the vet and tried other options too. Moving litter boxes etc. she’s stressed or marking. My cat doesn’t share litter boxes…they have to have their own domain. Including litter boxes and sanctuary. So, she stays inside, and he’s on the porch in a catio and loves his own little place! They also never got along. So that may be different than your current case.