kungfumaster

I have an SCCM 2007 infrastructure with two primary sites. I need to migrate both the sites to SCCM 2012. how do i go about it. I have already installed a parallel sccm 2012 site, specified the source hierarchy gathered data from the source hierarchy but now if i start migrating the collections and objects will it not bring all of them into a single sccm 2012 site.?? Assuming i migrate all objects from both the sccm 2007 sites to a single sccm 2012 site, how will i later put back the objects that belonged to the second site?? is it going to be done manually??