That is my understanding too. MW5 to scratch the Big and Stompy Solo (Coop) itch and MWO to scratch the Big and Stompy Multiplayer itch.
But while they have different engines, I would not be surprised if MWO (at some point) joined MW5 on the Unreal engine. As a matter of fact, I just may ask that question during the upcoming 16JAN PGI Q&A. : )
I wonder if PGI might do that eventually as well.
It does seem like the game is so far a long in it's life cycle that they might not bother with the amount of work it seems like it would be. At the same time though, maybe they are developing MW5 in a way that they could take a version of it and plug in MWO modules into that version, and make a new MWO in Unreal.
That would be interesting.