I ran into an issue some time ago where I needed to retain a history, and ran across the following:

Lee, Eric. "Merging Two Git Repositories Into One Repository Without Losing File History." SaintGimp, 2013-01-22. http://saintgimp.org/2013/01/22/merging-two-git-repositories-into-one-repository-without-losing-file-history/.

Hope that helps.