Or do you just have to leave the to figure it out amongst themselves?
I've just witnessed BiteyCat and next door's lovely puss have what I think may have been their first face to face encounter in the garden adjoining ours. There was no hissing or growling or arching of backs - in fact BiteyCat (bigger but more recent arrival to the territory, and on a lower wall because he'd been sneaked up on) was doing a lot of slow blinking to show he wasn't a threat, but he was also vocalising a lot (a low slow yowl) and when NextDoorCat finally retreated to her deck, he followed her and yowled outside her cat flap for a while when she went inside (it's microchipped, luckily, so he couldn't get to her.) I'm not sure if he was anxious or issuing threats.
They're both neutered, both feisty, but I'm keen to promote harmonious relations if possible, and definitely don't want him to become the neighbourhood bully. Is there anything at all I can do or do I just take my chances? I know if they were in the same house I could do scent swapping but I don't know if that would be advisable with them living next door to one another.