Even with the current structure you could have an OOB with Generals reporting to field marshals off-map. The FMs would supply logistical bonuses while the generals do the fighting. Currently what is the point of the distinction? FMs are simply uber-generals with different bonuses.
As described so far? Generals are Specialists, Marshals are generalists (pun optional). You put a Marshal in place when you want a general efficiency boost for an entire area. You use a General when you want to really get the most out of a particular kind of troops, or particular terrain. Marshals run the overall front, but you get a Fortress Buster and load him up with Arty or Assault Guns when you need to break through the Gustav Line.