I have produced twins twice. The first set were both female normals. I did keep them, and are raising them with the intent to brred them to find out if they are more likely to produce twins. They are three years old now, but are very small animals, and aren't the best feeders.
The secoind set I produced this past year, and both are male Mojave Het Hypos. I have them both for sale, and thus, may never get to breed them. Although I have thought of keeping one to breed to one of the twin normals, to see if it ups the likelihood of getting twins, if both parents were twins.
As stated above, it may be a couple years before I get eggs, but I will post updates when it does happen.
p.s. Great question.