If a client travels between the sites, it will simply use the closet MP to proxy its requests to its home site.
Is it possible to move clients from one site to other is case of disaster? Yes.
Is it a good idea? No.
There is no way that you can you easily reassign clients from one site to the other in bulk. It is a manual process.
Jason | http://blog.configmgrftw.com | Twitter @JasonSandys