Did you try removing everything you recently added, and starting with the one camera that won’t work at the moment. If that one camera won’t join, on its own, but the the other camera will, straight away, that should be sufficient to convince Blink that it is a camera issue.
If, on the other hand, it is always the second camera you try, whichever way round you do them, then that would point to something else. Some strange software issue, perhaps. Who knows?