The HD dish is made so it can look at three different satellites at the same time. It is wide because the three satellites are lined up sideways along the ecliptic. You adjust the rotation of it for each location because the straight line that the satellites are in is only horizontal at one longitude. You could get the same effect by aiming three dishes, each at the satellite picked up by LNBs A, B and C. The elevation angle would be tricky, though. You can look up where to point a round dish for Satellite A, but nobody lists the elevation to look at satellites B and C except through the combination dish.

Now to answer your questions:
Yes, you can use the multiswitch that comes with HD dishes with two round dishes.

Where would you aim the second dish? That depends on your latitude, longitude, and occasionally your attitude.

You will have to experiment to locate Satellites B and C.

How the heck did you end up with a switch but no dish?