Most new installations by Sky have them already fitted. I think they do it to east the install of Sky+ when they flog it (I mean "up sell") to you.
If you only have a singe a dual can be had on Ebay for a few quid.
We've got an Octo Lnb at the moment as we have 2 Sky+ and a Freesat in the Kitchen (no Freeview here.

) Sky through it in when we bought the second Sky+ I wonder if I could get some coax long enough to share a spare port on ours?