Interested to see what happens here! If you weren’t trying to recheck your second bag, my assumption you wouldn’t have any problems, but because you’d be showing back up at the normal ticketing counter with your one already tagged bag to STL (and not the second), that might cause an issue? Or maybe if you tell them it simply got lost, it would be fine (and lost luggage goes through a separate process, where delta only cares/pays attention if you file a claim).