Before the water saving changes made to toilets, yes, if it was clogged, a single flush could cause it to overflow. That went away for any of the new ones. The original thought here was would this save you from a leaking flapper. I suppose, if the second drain exited after an internal clog and you left the toilet plugged, and the flapper was leaking, then yes, it would save things. Anyone that flushes a clogged toilet when the bowl is already full expecting it to clear itself deserves the messy cleanup. Hopefully, they won't do that again! And, if the clog is downstream of the second drain, it won't help in the least.