I didn’t post this earlier because I wasn’t sure myself what it meant!  W were told during our NT scan that our twins were di/di.  I finally found a pretty good reference to what that means:

“Brandon and Brenda type twins are referred to as DiDi twins or Diamniotic-Dichorionic. Meaning the babies have separate sacs and separate placentas. This pregnancy can produce a boy/girl, boy/boy and girl/girl combination. Probably with the least amount of risk of the twin pregnancies, DiDi twins have less complications.”

More information from Wikpedia (http://en.wikipedia.org/wiki/Twin):

“Normally, twins have two separate (di- being a numerical prefix for two) chorions and amniotic sacs, termed Dichorionic-Diamniotic or “DiDi”. It occurs in almost all cases of dizygotic twins (except in very rare cases of fusion between their blastocysts^[33] ), in 99.7% of all pregnancies,^[34] and in 18–36%^[35] (or around 25%^[33]) ofmonozygotic (identical) twins.

DiDi twins have the lowest mortality risk at about 9 percent, although that is still significantly higher than that of singletons”

During one visit one of the OB’s said that our twins could still be identical even though they were in separate sacs.  I suppose that’s the very rare situation where a single egg cell divides in just the right way to separates the two babies.  So ours shouldn’t be identical and I most likely released two eggs that were both fertilized.