Actually, Donkey and mule twin foals are more apt to survive than horse foals. Donkey foals surviving are quite common compared to horses. In the ADMS publication "The Brayer", there is a couple of twin foal stories each year.
On the other hand, donkeys do get just plain huge when pregnant. My Hershey is fat all the time, but she honestly looks about 10 months along, when she is only 2! But the slimmer girls will almost get a sunken in look in front of the hip bones and their lower bellies will stick way out.
Good luck!