You have my worry also. I have a set of fraternal twins and would like to have another but I am not ready for another set of twins. Not to sound selfish but I would like to have just one after these to sit back and enjoy. My son is four and my girls are 3. So my son wasn't a baby long before I had my girls. I was overwhelmed. So I understand your concern. I know someone who has had three sets of twins...in fact I have another friend who had three sets but lost one of two of the sets and both of one set.
I have met a few woman who have had two sets of twins. I don't know what the chance is and there isn't ANYTHING we can do about it so just take what God gives you and pray for health even if it is two or three!!! Keep us posted....now I'm curious.