I found this picture from last week (Week 19) that I thought I should share with you. It shows the twins together in utero. From left to right (the three big blobs), what you're seeing is belly/ribs, head in profile looking up, and then head of other twin from the top, also looking up. The two little blobs that sit right above either side of the big head blob in the middle are hands: one from one twin and one from the other.
Isn't that amazing?