I thought I would give an update on my installation of my XPS 420 + Dual ATI DCT Cards.
Received both tuners. Connected them one at a time, upgraded the firmware to 1.17 and verified they worked w/out the cable cards. I only had one Cable Card, so after verifying both tuners worked, I pluged in the CC to one of them (left the other uplugged) and went through setup.
Called Comcast and paired the card with the ATI host. Didn't have any issues and was recording non-premimum HD content pretty easily. So far so good.
Went by Comcast the next day and picked up a second S-Card. Spent 8 hours trying to get the second card/tuner to work. Both tuners showed up in Network, but I couldn't get Media Center to see the second tuner. In all my things I tried, I also un-installed Trend, turned off windows firewall and everything else. Finally I got VMC to see the second tuner. However, it wouldn't tune anything. I finally gave up assuming the card was bad.
Went by Comcast again today, swapped the S-Card for a new one and picked up an M-Card as well. Got home, tried the M-Card and all non-premium channels worked. I think we messed up the pairing on this for the premium channels. Unpaired the M-Card and paired the S-Card. Same results as the second S-Card, where nothing would tune.
Called Comcast back and changed the pairing back to the M-Card (non-premium channles are better than none and since all the HD channels were working, fine). However, after pairing the M-Card and waiting about 5 mins, all channels are now working on both tuners.
I've got four OTA HD tuners and two DCT Tuners saving recorded tv to a striped pair of 500GB HDs. I've recorded a ton of stuff so far and no issues. Lets hope the 1.17 firmware fixes all the reported issues. Besides the PITA second tunner not showing up in VMC, everything else was pretty straight forward.