I assume by your question that you don't have the multiroom in situ.
Changing the LNB into a double or triple and running an additional cable is OK but you will have to register the serial number of the second box with SKY in order to receive a signal to it.
I put a double LNB(low noise blocker) on my dish and bought a second box via a local ad for �20. Having registered the serial number of the second box with SKY they said that I could either pay an additional amount ( I think it was �10 per month)to have my current set-up(sports) to the new box or pay a one of fee of �20 and have freesat. I did this cos I don't get digital in my area.
I hope this helps.