You will be lucky to get a double LNB, they just aren't around any more. You can get a quad (which will work just as well and give you spare ports) very cheaply on EBay. Ensure that you get one that is compatible with your Sky dish. (It will say so in the ad.) Don't buy from Maplins; they tend to be very expensive.
Before you do any of that, check that you do not have quad already as they are very common these days.
However in order to get the same Sky service on both boxes you will also need a 'multi-room' subscription AND BOTH boxes will need to be connected to a telephone socket.
If you don't take out multi-room, you will, still be able to View Free to Air Channels or, if you have an old Sky Viewing Card, Free to View as well. (Search here for more information as it has been answered many times.)
T
June 2009