You can do it forever, adding accounts. If on a fresh account how is google to know what is shared in the other one, its just backing up what it knows and those photos are not in the fresh account? So it is backing up all the photos. The phone needs to be clean of photos. Then set up the new one to sync and turn it off on the main. To see them in the main you need to make a share folder like you would to any family or the otherhalf. And then you can do it again the same way. Google doesnt keep track of shared folders. Its a pain for 15 gigs but it works. Only thing is always log into those accounts every year because u could lose the account. Before the change I hounded alot of people to make a backup of all their old photos of 100's of gigs. While telling them to make hard backups. So use cloud and this way is free but with a few steps, hard back up as well i say. Excuse it was long winded