Edited to say that 757 / 767 isn't O2T or MFF as it is a common type rating (like the B737-300 and B737-800)
As is the 777 and 787 both under FAA and EASA rules.
I know of at least one airline that got approval for 737/757 and 757/767 MFF, including training credits so that only one SIM event every six months had been enough on alternating types. However despite the approval it was never set into motion as the 757/767 left the fleet.
Anyway, as far as i know Lufthansa only does MFF on longrange types (A330/A340), they considered doing it with A340/A380 but decided the differences are too big. I heard somewhere that Swiss uses the same MFF mix. Finnair i believe has pretty much everything mixed, except A320/A340. Air Berlin uses A320 family and A330 but is considering about splitting longhaul and shorthaul depending on fleet development which is nowadays decided in Abu Dhabi.