Are you saying that you have a bicornuate uterus and are now pregnant and have learned that it is twins, or are you saying that you are wondering if anything about having a bicornuate uterus in fact causes twins? (If the latter, the answer is no.)
It sounds like, in your second question, you want to know how much risk there is to the baby from presumably being crowded in a bicornuate uterus. Hard to say without knowing what is happening in your uterus. My son just pushed the septum aside as he grew and filled in the entire uterus. It all depends on how big your septum is, I guess. The risk is not that the baby would be unhealthy, it is that there is some statistical risk of an earlier labor.
It's possible to carry twins in a bicornuate uterus and to deliver them at or near the normal term (keeping in mind that twins are often somewhat early). It would depend, again, on how the uterus is structured, whether the babies would be crowded by the septum or not.
If you are not presently pregnant, you could talk to your doctor about having the septum removed. Then you wouldn't have a bicornuate uterus any more, and wouldn't have to worry about the risks if you were to carry multiples.