If memory serves me certain channels require that the LNB on the dish is set to Horizontal and others to Vertical. With this in mind you need two cables because the dish has a separate LNB (all built in to the same unit) connected to each cable. Therefore with two cables (and effectively two LNBs) the box can decode and record both channels (regardless of the V or H requirement).
its' true some channels use vertical polarisation and others horizontal, but this is not the reason for two cables, even the sd boxes with single tuner receive channels on both poles, they do it by voltage switching between 13v/18v which in turn switches the lnb between vertical and horizontal poles.
the reason for twin lnb/two cables is that in simple terms a sat signal cant be split like a terrestrial aerial signal just with a splitter.so basically as pvr's have twin tuners you need two feeds.
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