I'm wracking my memory here to try and recall which games have used a similar "shared unit" mechanic... Perhaps Warcraft 3 did, if Starcraft did. I can't recall, but it's possible the Earth: 2150 series of game did, as well.
For dedicated co-op campaigns, you could check out Red Alert 3 (OK game, shitty DRM) or Starcraft 2 (Great campaign, toxic multiplayer). I don't think either has the unit sharing you're looking for, but it's something.