Split the cable before it goes into the cable box, and run one leg to the cable box and one to the TiVo. From the cable box, run composite or S-video to the TiVo.

Note: this only works on Dual Tuner units.

I might have not made my self clear. My cable box only has the single coax output with no extra composite or S-video outputs.

Oh. Then there's no way to do dual-tuner recording in that situation. You can probably ask your cable company for a new cable box.

This is my own cable box. I do not want to pay for a new one. The TIVO specs make it sound like it takes the coax feed and has a splitter. Can I not crack the case and get to the output of the splitter?

Nope - the way it works is that the internal tuner has to have a direct cable feed to split it. It can't be split to one cable and one post-cable-box, at least as far as we know. And even if you could do it, I don't think there would be a way to configure the software to recognize it.

Possible Solution

I know I'm waaaay late to Tivo-land, but I thought my answer might possibly help someone else.

With the 9.3.2 code, you can get single-tuner capability through your cable box (or Digital Transport Adapter - DTA) that only has a coax output. With a lot of cable providers going digital, you must have some type of box on a S2DT in order to get anything. Since the Tivo can only control one external box, you're basically left with a single-tuner Tivo. Better than nothing, I guess.

Here's how to get this to work:
Go to Settings->Video->Video Hookup

1. Say Yes, I have a cable box
2. Say Yes, it is digital
3. Say No, I don't want to enable dual-tuner capability
4. Confirm you do not want dual-tuner capability
5. Exit Video Hookup

You should now be able to hook the coax input on the Tivo to your cable box and change channels from the cable box (or DTA). I know for a fact that the Tivo can control the Thomson DTA in this manner, using the Tivo remote, if you have the proper IR connection.

Hope this helps someone, sometime.
SKaL