going to 2 dishes is always the better option because you can fine tune the dish to only one satellite, instead of compensating for 2 satellites
Yes you csan go with 2 31" dishes (although depending on package, you may only need it for one satellite). I had a 24" for 119 and a 18" for 110 (I had AT60+ locals so 110 was only needed for my locals).
As to the LNB, easiest thing to do is the LNB holders will accept a 40mm LNB (normal KU LNB) but also has an adapter for a 20mm LNB. I busted the case off my LNB and the LNB fit right in there with the 20mm adapter
