The reason you are unable to to do this when using a cable, satellite, or DTV converter box, is that most cable and satellite boxes, and all DTV converter boxes, can only download one channel at a time through a single cable feed. In other words, the cable, satellite, or DTV converter box determines what channel is sent down the rest of the path your VCR, DVD recorder, or Television.
If you have a Cable TV, Satellite, or DTV Converter Box and you still desire to be able to watch one program, while recording another, you have two main options:
1. Purchase or obtain a second Cable, Satellite, or DTV Converter Box. Connect one box to the DVD recorder and the other to the TV directly.
2. Inquire with your Cable TV or Satellite Service if they offer a cable or satellite box that has two onboard tuners with separate outgoing feeds that you can send to the DVD recorder and TV separately.
NOTE: Your TV needs to have both an Antenna/Cable connection and AV input options, as the cable or satellite feed can be connected to your TV's antenna cable connection, but your DVD recorder would have to be connected to your TV's AV inputs to allow the playback of recorded DVDs. If your TV does not have both AV inputs, in addition to an Antenna/Cable connection, you will have to purchase and RF Modulator to be able to connect both the cable feed and DVD recorder to your TV.
Proceed to the next question: Can I make a copy of a Cable/Satellite DVR or DVD recording that I made from HBO?