The LNB on the dish can't send all the channels down the co-ax at the same time - a signal fed backwards from the receiver tells the LNB which group of channels to select. Similar to horizontal and vertical polarization on terrestrial aerials, but switchable. As long as you only want to receive one channel at a time that's fine - it just switches to the appropriate channel group. If however you want to watch one channel and record another or record several different channels at the same time there's a problem as not all the channels you want might be in the same group - hence most recorders have (at least) two co-ax inputs and the LNB on the dish has two (or more often 4 or even 8) co-ax outputs - each cable capable of carrying different groups of channels at the same time.
