They have what's called the SWMLNB which can support up to 8 tuners via a single coax, just using splitters where needed for distribution. The key is getting DirecTV to give you one on the install (don't bother telling the CSR you need it, they ignore the request). Last I heard is that if you order at least 5 tuners and HD that you can get the SWMLNB. So if you order two HD DVR's and a non-DVR box, or one HD DVR and two non-DVR boxes you should get one.
My experience, with my son and his installs, 1st install was one HD DVR, 1 HD STB and 1 SD STB, installer didn't want to do a SWM LNB. Showed him how easy the install would be with it since he could use all existing wiring, and if they didn't the install was cancelled. He called the office, got the OK and he got the SWM LNB. Three months later a move to a new house using the movers connection. Again, no SWM LNB on the order, again showed the installer how easy the install was due to existing wiring if he used one, again he called the office and got the OK.
If you don't feel lucky you can always order a SWM LNB on your own and ask the installer to use it, but I've seen a post where someone tried that and the installer refused to do it and walked away.
So bottom line is that it's possible to use one wire to HD DVR's, but YMMV trying to get D* to install one.