I ended up making two of them in one style and then I thought it might be too long and voluminous if the babies were preemie-sized; so I made two more in another simpler, shorter style.
I used one of my porcelain dolls (who happens to look a lot like the twins' father when he was a baby) to model the two types of dresses. I think she looks sweet in them.
Making christening dresses out of pillow shams is exactly the sort of project my mother-in-law would enjoy and appreciate. I miss her and wish I could show them to her!