I had a chance to talk to a UPS guy and asked him about the "wing-tip" 757's that often operate SDF-CLE-SDF. Specifically I asked whether a shortage of wide bodies was the reason. (The MD-11's weight and cube capacity  almost exactly equals 2 757's.) He said no, that the dual 757 flights offer UPS the option of upgrading one of them as an incremental increase in capacity.  (A 757 and an A300, I guess.)

Does anybody know if the 757/A300 pairing has actually been flown yet?

Edit: I see that tonight's operation in an MD-11.