The issue isn't the number of networks, but rather the number of frequencies used. The second ssid being broadcast will probably run on the same frequency as y

The issue isn't the number of networks, but rather the number of frequencies used. The second ssid being broadcast will probably run on the same frequency as your usual ssid. Switching to super high frequencies will mitigate the overlap, since these won't travel through thick walls, but then you may need to put wifi points in all you rooms.