What Mr. Goodwin said, I have the same setup (with the FireDTV) and you can only watch two channels if they are no the same stream. Most digital broadcast stations use 10 streams or so. The 10 streams contain 100 TV channels and some digital radio channels.
You have to know which channels are on the same stream. The FireDTV/FloppyDTV should be able to render all channels on one stream at the same time.
For you reference, I use Multikabel in the Netherlands and I can watch multiple channels at the same time.
Please look here for an overview of the Multikabel streams. Your provider should have a similar list somewhere on their website.
http://www.digitalekabeltelevisie.nl...ltikabel.shtml