This sounds a lot like a proposal that was kicking around our Powerplant group a few years ago. The basis is that a high bypass ratio fan is great for takeoff thrust, noise etc but its efficiency is much less than a pure turbojet at altitude.
Guess the problem would be what do you do with the other half of the pair when you don't need it - don't think any (civilian) pilot would intentionally shut down his main engine twice in a flight...