I recently enabled iCould Document folder sync across my two macs (running Sierra). I share the sparse bundles using dropbox across both macs. I also have folders inside of the iCloud-sync'd Document folder that I use as mount points on both macs. So far that seems to be ok but I want to confirm that it officially should not be a problem, particularly if I have the folder unlocked on one machine and iCloud tries to sync the mount point location. I don't seem to haver a problem so far... :P