They may be genetically identical in terms of human DNA, but most likely the bacterial DNA of the micro flora in their body is different. In addition to this, I seem to recall that in a study of zero gravity, bacteria flourished due to the lack of resistance naturally caused by gravity. They have already tracked some long-term results, which include loss of bone density and other physiological changes. I guess that's an interesting case study that might be useful until we're able to send large numbers of people up and get a better statistical analysis of what happens to people in space. Still, on human development these effects will be negligible. Since identical twins are not identical. It would be more interesting to perform this experiment on children who are in their growth spurt. Since mechanical stress has a large impact on bone formation. Not the most ethical of experiments though.