There's a post on the boards somewhere here about how it works. In short, there's either a medic in range of a given injured unit, or there is not. If there is, the unit heals an extra amount. Having multiple medics (of either type, range 0 or range 1) makes no difference at all. There is an upside though, given the promos are there, we can now promote with March. Ordinarily I don't do that, as I'd prefer two "attack strength" promos. However, once you've got Medic, I love the March thing because it means those units never have to stop. It significantly increases their power. March means a unit can do anything in its turn, and still heal. If a medic is around (at the end of the turn; actually healing happens at the start of the next turn), there's more healing (as per above). If a unit lacks March, it basically has to sit still during its turn to heal.