I think it is impossible for a team to be completely dominating in both areas, just as US Postal did in the mountains for Armstrong, and Fassa Bortolo or Saeco for Petacchi and Cipollini in the sprints. What is possible is a mixed team with mainly mountain men working for a GC contender, and one sprinter with maybe one leadout man. Think Rabobank with Menchov and Freire, or Silence with Evans and formerly McEwen. But those are sprinters that don't necessarily need a leadout train, so it's a different thing, it's not dominating both fields.
It's just a question of numbers in the end. How much men does it take to completely dominate a sprint stage? At least two during the stage to chase breakaways, and perhaps 4 or 5 to control the bunch in the last few kilometres and form a train for the sprinter in the last 2km. That's at least 6 or 7 guys, who will have to repeat this trick like ten times during the course of the GT. It goes without saying that you can barely use those guys in mountain stages, let alone keep two or three to drag your climber over the biggest cols. Same goes for a mountain team. And the climbing types that drag the peloton all the way up the Tourmalet are usually not very usefull in a sprint leadout anyway. Jens Voigt being the exception that confirms the rule