neighbours' kitten keeps sneaking in my house

Hi, my first post here. My neighbours recently acquired a kitten who is approx 10 months. He seems to be 'friendly' with my neutered tom, Milo, however, as he's got more adventurous and curious, he's started following Milo into our house (just in the last two days). I don't have a  catflap, and so Milo, whom I rehomed as an indoor cat, gets time outdoors only when I am at home and can leave the back door open.  I've witnessed Milo bossing around other neighbourhood cats who've strayed onto his territory; now, however, Milo is having to share that territory with the neighbours' kitten. The neighbours' kitten appears submissive to Milo, and meows at him kitten-like, though I've also witnessed a few mild sparring sessions, and I'm never entirely sure who's won. This afternoon, however, the neighbours' kitten sneaked in and went into Milo's litter tray (which 'd just cleaned this morning!). He didn't poop, but perhaps peed. I've changed the litter again but am wondering: is this kitten's behaviour aggressive? Is he trying to dominate Milo, on his home turf? Do I need to be wary of my interactions with the kitten, and Milo's?  I've locked the door now, and decided that when I let Milo out again, I  will have to lock him outside as I don't want to risk the kitten coming in again. What do you think of this kitten's behaviour? Can two neutered male cats be 'friends' or does one need to be on top? 

Your opinions would really interest me.

Thanks!

my neutered males have always gotten along very well but of course, they live together. it might be best to keep the neighbors cat out of your house to avoid any problems.

Yes, neutered males can become very close. I don't see any signs of aggression on the part of either cat. The kitten just wants to be friends with your cat. If you don't mind him being in the house, leave the door open and let him visit.

Thanks. I don't have any experience of kittens as I got Milo as a three-year-old. I'm not sure if the kitten is just curious and snooping around Milo's home because he wants to get to know him, or if the litter tray thing is a sign of him attempting to 'bully' Milo. Of course, it may be that Milo has strayed into the neighbour's home too, and this is just payback! They got their kitten because they loved my cat so much - chose the same breed. I just don't want my boy to feel unhappy in his own garden or home. For the time being, I will keep the kitten out and just let them play it out in the gardens, rather than my house.

Thanks!