The problem is that external STB's may have two tuners but they can still only output the signal from one of those tuners at any time - you would need 4 dual tuner STB's to deliver the capability you describe I'm afraid.
Some Dish Network dual tuner DVRs (such as vip722, vip722k, etc) support "two rooms" and therefore, have two video outputs. Not sure if you will be able two control two separate "rooms" with IR, though. Typically TV2 responds to the RF remote. I have successfully used two RF remotes before, but not two IR remotes. Maybe setting different addresses for each is sufficient, but I've never tried it.