Keep connecting them. They will eventually be fireworks and they will have babies. It shouldn't take too long if you constantly connect them, but it might take a while, so you will have to be patient.
Yeah, just keep connecting them together, eventually you will have 2 Tamagotchis that are mating. This does take some time though, so you will need to be patient.
Also, use the love potion, it speeds up the process of mating, meaning your Tamagotchis won't need to connect as much.