Yes, the SWM8 does need two coax cables to feed it from the antenna. I forget exactly what the voltages are, maybe 13v and 18v, but don't hold me to that. The SWM8 sends 13v on one cable and 18v on the other. That puts the even numbered transponders on one cable and the odd number transponders on the other cable. That makes all of the transponders available so you can get all of your channels.
The non-SWM handles that by switching the voltage to the antenna from one voltage to the other to select the correct transponder group for whatever channel you want to watch. The SWM8 does not allow that voltage change to propagate to the antenna.
FWIW, I have an 18" round antenna on my S&B house feeding a SWM8. I don't have HD service so I don't need the 3 LNB or 5 LNB dish, but wanted SWM for single coax service to a DVR. You will probably get an error message that the receiver can't find 1 or 2 satellites. That is correct, that antenna won't see but 1 satellite. You can ignore that error message. It will work just fine.
P.S. I hate that Bill.Satellite is no longer here. I learned a lot from him.
P.P.S. I probably should have put that 2 coax requirement in my earlier post telling how to convert your rig to SWM. I just didn't think of it.