To add to hypergotchi's information, that only happens if you have 4 hearts. You can't just make 2 tamagotchis that do not know each other that well kiss. That would be awkward.
that is very good advice. oh and ive tried and its very hard to get a v3 and a v4.5 to mate. so i just wanted to let you know. have a nice tama talking day. and have fun with your tamas!