one way you can kind of have twins is when you connect with another tamagotchi pet and they have babies. Whenever I connect two tamagotchi pets together and they have a baby the always grow up being the exact same thing as the other.
Well when u think about it your tama can have twins exept u only keep one. Thats the only way i know of haveing twins.
MY CONCLUSION:
You actully do have twins but you only have one of the two ttwins other wise your tama would have to have more than 2 babies to give each parent twins!