If the two you have right now are a boy and a girl, when they have bothe turned into adults, they can have babies..
These babies will be of the same gender on both tamas and after the parent has left become the next generation..
Once your adult tama has had a baby, it cannot have another one, it will simply leave you to return to it's home planet and you to care for the baby, that baby will then be able to have a child once it has grown up..
If you do not have a tama of the opposite gender to have babies with, the matchmaker will come along when your tama is around 7 years old and she will offer you a partner to have a baby with.. so NO it is not the end of your generations when you end up with two tamas of the same gender..
Hope this is of some help to you