Yes, morulas can split into identical twins. The split generally happens anywhere from 3- 10 days after conception. Of course the later the split the higher risk(siamese or MoMo twins). My ID twins were not seen by RE at 5w1d in fact, it was clearly just one sac. Much to our surprise at 6w3d, the ultrasound tech at OB's office saw them very clearly :) Still a very real possibility, I would say. Hang in there and hope everything continues to go well for you.