Bob, originally when the designed the box they inded to have the sat & ota tuners recordable at the same time (up to 3 or 4). THe first version of the HR20 manual even referenced 3 recordings at once (I have 3 copies of this manual).
If I remember right, when they launched OTA (five months after the HR20 launched) they found the processor could not handle the load of 3 HD recordings and doing playback, so they never went there.
It would be nice, but we'll never see it, especaily considering the HR21 works to move away from OTA (excluding the add on box).
Since the HR21 has a faster processor, I wonder if they will consider allowing more than 2 recordings for a HR21/AM21 combination?