The receiver you connect to the satellite dish (LNB & Diseq included) is able to tune to one channel (or frequency) at a time. That is why you will be able to watch only one channel at a time, because the tuner cannot tune to two freqnecies at the same time.
In the case of your uncle, who receives it via cable, i believe (you have mentioned not by Satellite), the cable operator puts different channels at different frequencies (UHF and VHF) on the same cable. To watch these channels, you have to just connect the cable to the TV directly (no receiver) and tune the TV to the given frequency. As TVs comes with their own tuner, you will be able to tune each TV to a differenct frequency on the cable.
Back to satellite, even if you have two receivers, they will not work on a single dish system simultaneously. This is becuase of the different voltages and frequencies needed to energize the LNBs and Diseq switches. When two receivers try to access the same Diseq and Lnb with different paramets, then there may a damage to the equipment involved including receiver, Diseq switch and LNB.
Hope this answers your question.