My understanding is that the eggs 'hatch' together because it would be largely dictated by the hormone levels in your body. However, under artficial treatments like IVF or clomid, I could understand how eggs may come down at different times.

I saw a show that had a mum delivering one twin prematurely (about 26 weeks) and the other one staying inside for the 9 months. SO even though they were twins, they had different birthdays (by about 3 months). But I believe they were conceived at the same time.

Hayseed