A DBS box splitter contains a few diodes that prevent the various boxes
from each feeding the downconverter supply (only one does). There are
usually instructions on which port has control. The other port is limited
to see channels with the same polarization (as selected by the first box).

good luck,

Peter

PS: Check everything twice! There is a very good chance that you fry the
downconverter and/or the boxes if you are not careful. Also the signal
strength may not be enough to feed two boxes on one singal. It depends
where you are. You may need a wideband amplifier. Schematics have been
published (The amp needs to amplify cca 0.5 to 1.8GHz by 6-12dB with Nf <
5dB - this is trivial nowadays - f.ex BFR90 etc). The 'subscription'
usually applies to seeing one set of channels on one set ;-).

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