Congratulations tabbycat. My twins are 17 now, so I am not really the person you are looking for. However, it is a joy and different to raising singletons in lots of ways.
I do remember being completely stunned when I was told I was expecting twins. I was so sure I was in touch with my body and knew I was pregnant before I even missed a period, could not believe that there there could be two creatures alive inside me without me knowing, when I had so clearly pictured one. Initially it made me completely mistrust myself (as in "if this is true, what else do I not know about my body/ my baby. How can I feel so confident everything is OK..."
Everything was OK, but it took a little adjusting.
Best advice I can give you - beg borrow or steal a copy of Elizabeth Bryan's book Twins, Triplets and More which is fabulously helpful. Sadly I think it is out of print now, but it may be available second-hand.
Explains lots of stuff that is different with twins, for example, that they are each other's primary attachment object (you will be a secondary attachment object ie honorary dad). This means that at 6 months they get separation anxiety from each other, rather than you. In my case, that meant that if one of them cried and I picked him up to comfort him, his brother would cry, not to be picked up, but for his twin to be put down.
Good luck, and enjoy