If this is a duplex with two owners, then as the owner of the other unit, I am not going to permit you to come onto my side to make this connection.
Don
They maybe they dont let you access that sewer clean-out on thier side when you need it?
Either way the connection needs to be made <5' from entrance of the
building, if the two units are the same structure then they are treated as one. I would also assume that it has only one service entrance, and only one water feed?