You can mount a DirecTV LNB on a Dish Network dish, but the reverse is not easily done. If you want to use a DirecTV LNB as part of your Dish system, there's no good reason why you can't. I've done it, it works fine.

But if you mean to receive both Dish Network and DirecTV services with the same LNBs from one or more dishes, it's a more complicated question. With the right equipment, you apparently can. See: http://www.techweenies.com/dss-1.shtml for diagrams and photos of several configurations that are possible.