Well, I ran and SD PVR and then an HD PVR (and then both at the same time) off a standard dish (which I believe is 60cm). While you'll get better signal with a bigger dish, it'll only count for something if you trying to watch TV in a thunderstorm.
So no, I can't imagine you needing one. But nothing prevents you from just hooking it up right now. If it turns out you need a bigger dish, get one then.
A multiswitch is a device with two inputs (from your LNB) and 4 (or 6) outputs that goes to your decoder(s). You mentioned you've got a dual LNB, was both ports wired all the way down? So currently, one cable is disconnected coming from the dish?