Nice idea. Actually, I had just started making a mission similar to this a few weeks ago. Well, my idea was a little different, and looks like this:
Basically, it would be a campaign that consists of a loop of two missions. First, you would have a strategic planning mission, where the map is divided into squares, and you decide where you want to move your units. A 'unit' would be a platoon of 3 groups.
After that, you would go through a series of tactical battles; a separate mission for each area of the map where enemy and friendly platoons are occupying. At each battle, there would first be a setup phase, where you place your units (either on the border of the zone if you are attacking, or anywhere inside if you are defending). Then you would press a button, and the RTS style commanding would begin.
After that full cycle, then it would be the other side's turn, and they would have the same cycle as above.
This type of game wouldn't really work in MP, because, well, turn-based strategy (TBS) just doesn't work well in real-time. The way I've laid things out, it would seem like it would have to be single-player vs the AI. Not true. I was thinking play-by-email would work well. Basically, you both have the same campaign file. You take your turn, and when you fight the other guy's units, well the AI will have to fight for him. Then you email the save file to your buddy, and he takes his turn. Passwords could be set up to keep everybody honest.
If you really wanted to get complicated, you could have play by email for the strategic phases, but then for MP, there would be a separate MP map. When there is a battle, a little string of code would pop up, which you would copy and paste into the MP map and it would set up your units and the zone for you. Then you play the battle, get a new string to copy for the results, and paste it back into the campaign. But this would require structuring the turns quite differently than the first idea, and so I decided to go for the first route (play by email; AI acts for the other player).
Anyway, I actually fleshed things out with lots more detail in terms of game-play, but that is the general idea.