Not with the 508. It is a single tuner PVR, so the one tuner is being used to receive the program that is being recorded. Therefore, you have no other tuner to use to receive a different Sat. Channel than the one begin recorded.
You'll need a 2-tuner (also referred to as dual-tuner) PVR to do this. Dish PVRs such as the 625 and 622 (HD) (or the old, discontinued 721 & 942) have 2 tuners & you can watch a different program than the one you're recording.