Yes. You can synchronize certain files, like document files, between any OS that your Mac can copy to. For instance, you can synchronize your Documents between Mountain Lion and Yosemite or even Mountain Lion and Windows XP. However, you can only perform a Home-to-Home sync between the same version of macOS, meaning you must sync Yosemite to Yosemite but not Yosemite to Mountain Lion. Furthermore, you need to make sure that your iApps like iPhotoiTunes and iCal are the same version.