It's nice to take for granted that when delicate medical intervention is required, your native country is the place to have it performed . . .As to the issue of possible commonality of brain tissue between the twins, a recent and very impressive book (The Mind and the Brain) suggests that that is likely to prove not to be a problem. Although brain functions can normally be mapped to fairly specific areas of the brain, the brain itself is amazingly "plastic". If it be necessary to excise even half of the brain of a small child, the other half can adapt so well as to manifest no outward sign of the loss.
Even the adult brain adapts quite a bit; a stroke victim is well advised not to give up but to attempt to regain function--it seems that the effort tends to rewire the brain around the stroke damage . . .