hmm... could be an age thing.. I mean maybe he feels she´s older but still want to let her know he was there first and is tops.. even if they get along just fine.. just as a reminder.. I think..

the other pooches are younger so he might not need to let them know that.. and with Zoee he knows she´s top of him (so to speak)..

it does seems LB inclusion was the trigger as he´d never peed before her.. and he only pees at "her" house not on the other pooches house...I don´t know, maybe he just want to make sure she knows he was there first..

I really hope you can sort it out and no more peeing in the inside garden.. lol