Nope. You can watch a previously recorded show while recording a live channel, but you can't watch a live channel while recording another live channel. At least not with the 510.
Timohilo- The 721 is the way to go. But, it's over $300, if you can still find one at a retailer.
The 522 can do it, but: A. It's still quite buggy, and B. It seems to be available only to new lease customers.