First off I'm not a doctor (although I sometimes play one........... uh nevermind)
But after looking at the condition on
Web MD, it looks like this is something that happens during the pregancy. Because identical twins have the same genetics, and are exposed to the exact same environment during gestation, I would say that if one gets it odds are both will have it.
Now, is your question based on knowing one has it, what are the odds of both ending up getting it? Or is it a simple question of what are the odds out of all identical twin births for this to happen?