It will work with one line but you'll only be able to record one show at a time. You need two lines to any DVR to get full functionality.
What kind of house do you have? 1 story? 2 story? Basement, finished or not? Crawlspace? Depending on your situation it may not be very difficult to run an extra line.
The other option like the previous person said is to wait for the SWM (single-wire multiswitch) to become available, it's being beta tested now but no foreseeable release as of yet.
If this was installed by a technician, and he didn't run the second line like he is supposed to, you do have some recourse. If that is the case I would call Customer Service and tell them the person did an incomplete install, shouldn't cost you a dime to have someone come back out tio run that line. Keep in mind a standard install, as far as running cables is concerned, is either drilling through the floor to bring them up from a basement or crawlspace, or drilling through an outside wall directly into the room. If you would prefer the extra cable to be fished up/down a wall there may be extra labor charges to do that as it is considered non-standard work. Hope this helps. ^^