There seems to be some misunderstanding here.  I didn't suggest you share a separate build directory between repositories... I suggested you have a single repository that is the one you are currently building in, and that you synchronize it with various other repositories as you swap "branches".  That process should touch the modification times on the files that are changed, such that your partial builds are as trustworthy as partial builds are likely to be (that is, not very... but passable).