Um, ? Tosakotchi is an adult and Puchitchi is a toddler. Only 2 adults can mate.
They have to be compatible and they have to have been both adults for at least twenty-four hours. Keep connecting them until they fall in love or use the love potion. They obviously also have to be the opposite gender. Eventually they will mate if they are really in love and compatible and you keep connecting them for present if they are the same version. If they aren't then just keep connecting them.