First, you can't use receivers at both homes simultaneously, it's against your DirecTV terms of service - only one location at a time.
Call and ask to change your service address when you move to the other location. Then when you come home, change your service address back again.
A couple of things. First, you will get whatever locals are delivered at your new service address. Second, unless your dish at the "new" service address is an HD one, you won't get HD signals. Third, if you take an HD DVR you will need two cables run from the dish to the DVR. Do you know what type of dish is set up there?
The other possibility if you don't have the correct dish and wiring at the new location is to use the DirecTV Movers Connection. DirecTv will set up whatever you need at the new location, you just need to take your receivers with you. You can still change your service address back to your "old" house after three months.