Combine two regions in 3D

Hello, everyone,
I have two 3D meshes called mesh1 and mesh2, they have a common border s. Now I define a new mesh by mesh = mesh1 + mesh2,so s is an internal border of mesh. I wonder how to merge two internal regions of mesh1 and mesh2 into a region so that I can use tetgreconstruction to refine grids of the whole region.
Thanks for all reply.

I find extract can delete internal border,it can solve this problem.