Oh sure, absolutely. There is absolutely nothing in the contracts you sign with either DIRECTV or DISH that say that all your receivers have to come from the same dish. So if it’s easier, put a separate dish on your garage or guest house, or put one dish on each end of the house to save on wiring.
It’s worth saying, though, that if you use Genie or Hopper, you won’t be able to put the client boxes on one dish if the DVR is on the other. It just doesn’t work that way. But you can certainly put a standalone receiver or even a DVR on that second dish. You just can’t share anything between dishes.
Personally I would recommend making both dishes the same unless you are 100% sure that the only thing you’re going to run on that second dish is an old standard definition receiver. If that’s the case you can easily get away with an older-style round dish which is going to be easier to aim and easier to mount.