Reply to DmbFan1 and Interactive: What I believe is the case is that the second tuner on the STB "works" if you can watch one show while saving another. I had to learn this without the aid of a manual, since Comcast does not see fit to give their customers manuals for these devices. It seems that the two-tuner feature of the 6412 does not allow for the box to *send* two different programs though its outputs, but rather just to *receive* two different signals through its input lead, which allows you to save one show and watch another via the STB.
The advantage of using a splitter with one line going to the STB and the other going to your TV set (or your could go into your VCR and then out from their to the TV set via either red/white/yellow cable or antenna cable - or both - which allows for PIP using your TV set's internal PIP feature. You can watch non-digital channels in your TV's PIP window and the signal from the 6412 in the main screen and swap between them. If you do go through the VCR, then you can record a show that is on a non-digital channel through your VCR while saving an HD show on your 6412 and watching a third program in HD on your TV set. And, you can monitor what the VCR is recording and/or surf non-digital channels via the TV set's antenna cable. Mind boggling, eh?
But this works only if the splitter does prevent the 6412 from communicating both ways with Comcast. I had to remove my splitter, so no PIP at my house for now.
If I am wrong and the 6412 does allow for you see both the program being saved and a program that is being currently broadcast, at the same time via a split picture or PIP feature, please tell me.