To use the full facilities of Sky+ (eg watch one channel and record another), yes, you do need two LNBs (and two cables), but it will work on only one LNB - the dish I use in the caravan has only one.
You can purchase a card from Sky for a one off £20 (IIRC) which will give you all the FTA and FTV channels. Sky are desparate for customers, so if you want to use the full facilities of Sky+, and get the other channels, then take out a contract with them for whatever mix you want.
Give them a call and see what they say. Argue negotiate with them - they will adapt. I had a problem 12 months ago, and they said they wanted a minimum of £80 to send an engineer round. I told them I would cancel the service, and an engineer appeared FOC on the doorstep next day. Another time, I got hold of another Sky+ box for the 'van, but as the card is configured for each box, it didn't work very well. They said I would need to take out a second subscription but, after I again said I would cancel, they fixed it that both boxes would work
with the same card. My argument was I could only use the card in one box at a time - what difference was it to them if it was in two different boxes!
Del