Well, I think it´s possible to use Shaw´s elliptical dish (I think it´s the 75) but that´s from unconfirmed reports.
I also understand most of us here in the city already had larger dishes we used for Dish Network and after giving up hope with signal loss couple of months ago, have put those dishes now to better use with SD.
So I see that many of use are using 2 dishes one for each sat, I know that using 2 separate dishes even the 3 footers used locally by Sky are enough to get the signal.
Now if you already have a huge dish, I´m not sure if that will be good to get the 2 sats, but in such case you would need to have 2 LNBs, (one for 107.3w and the other for 111.1w
I´d say you´d get very good results using 2 separate dishes at least 4 footers.
A site´s member was using a 4ft for A F2 which carries most of the HD channels but he reported not keeping them steady, so he upgraded to a 6 footer with fine signal, I myself have a 6 footer for such bird and reception is still very good even with the recent rains.