How can I map two dissimilar objects to the same UV border? I am trying to minimize texture seams and distortion in order to use a tiling detail normal map on top of a diffuse map on a humanoid mesh for use with Miku Miku Dance.  I believe that if I use two different UV maps sharing the same silhouette, I can avoid visible seams and that the distortion will be acceptable.  I have separated my model into a front and back half (two different objects) rather than seaming it in Blender while trying different solutions.

Because the front and back half have dissimilar surfaces (but share a border), Blender UV maps them to different silhouettes, which will create visible seams on any map tiling over their surface.

Is there some easy way that I can get Blender to use the same silhouette for both halves?  I would prefer not to join and reseam my mesh, but if that's necessary, I'm willing.
Nathan, in fact it's more often the other way, meaning if you have two UV islands of the same model that share the same texture space it's much harder to avoid visible seams, even if those UV islands are exact copies of each other. It's quite impossible to have no visible seams at all if the two UV islands are only somewhat similar. 

The next big disadvantage of sharing texture space is that it's nearly impossible to paint away the seams, since you'd paint both parts instead of one.