Pretty much every DVD recorder will let you record one channel while watching another (same as video recorders), because you record from the tuner in the DVD recorder while you watch from the tuner in your TV, all will record while the TV is switched off (unless of course you want to record from Freeview and your Freeview is built into your TV), and most support Videoplus which was the video community's attempt to make recording as easy as possible.
...however, I suspect that doesn't answer all of your questions. Do you mean record to DVD whilst watching a DVD (DVD-RAM recorders support this technique, but there aren't many DVD-RAM recorders on the market)? Do you mean record from Sky or record from Freeview whilst watching another Sky channel or Freeview channel - unfortunately that can't be done, but that isn't the fault of any DVD recorder, that's the fault of the digital TV box not having the ability to output two separate signals to two different scart connectors. Many Freeview boxes and Sky+ can now take two separate inputs, recording one to hard disk while you view another, but I don't know of any that will output different channels to two separate outputs.