I bet it's probably dominant behaviour. When I adopted my little mixy, Kiki, he took a liking to my dad. He was not fixed at the time, so he would mount my dad's leg. He would mount other dogs too, but not aggressivlely, becasue he thought that the other dogs were females. Lol. It's doiminance. Becasue you already had a dog, your firstdog has claimed the house as his, adn any other dog in the house, is like an intruder.