The issue with Satellite I.e. Sky, is not a specific tv card, rather the skybox itself. As I understand things, they will only process the signal from one channel at a time, thus to record two channels simultaneously, two skyboxes are required.
You can use multiple tv cards simultaneously, using the proprietary software. However, the one catch is in xp versions of media centre - to use more than one dual tuner, you need to use a registry hack. I'm guessing you don't have media centre in mind, so no problem.