That is correct. MythTV runs a MySQL database called "mythconverg", and in it your scanned channels are tracked by their channel number, the PC card/device, and that card/device's inputs if there are more than one (such as RF, S-Video, Composite, etc.). If your system can access the same channel over two or more such cards, the software allows you to prioritize or change the weight of each card/device to prevent conflicts amongst scheduled recordings.
I was partially right though on the rest of what I said... The channel changer script is settable per tuner but is not settable for each tuner type...
For example, it is settable for Hauppauge PVR-x50/500 cards but not for a SiliconDust HDHomerun and it's not directly under the tuner config, it's under "Input Connections" (at least that's what they call it currently but I'm not sure it was always called like that).
BTW, is there such a thing as a Matrix antenna switcher (some kind of switch that lets you chose which antenna you use on a tuner/tv basis) and preferably remote controlllable?
Thank you very much for your help!