sorry, yes, Rolf is right, it would be a good idea to get 2 different manufacturer's receivers! Fortunately, there's many cheap FTA types. Otherwise, you need a dish big enough for the channels you want, plus a dual output lnb.
Where from depends on what's convenient, but for most people that's by courier delivery (Shops are usually more expensive, Maplin are very expensive!). Good prices at:
www.cpc.co.uk .
Packages have just a 1 output lnb, so then you'd to replace it with a 2 output one, and probably wouldn't save any money!