The DiSeqC really needs to be between the motor and lnbf. While it might work between the receiver and motor it will not work long. I just burned one out doing exactly that. I have multiple dishes feeding a single FTA receiver and was using the DiSeqC inside the house. Worked fine for about 5 mos or so. Still changes channels correctly but no longer passes enough current to drive the motor. Kinda doubt it will change polarity correctly either
I have come to the conclusion that if using multiple dishes you really need an A/B switch of some sort. I have yet to find one that works for a satellite system. Honestly I have not looked that hard. Bought one at Radio Shack and it did a fine job of stripping signal
Sadoun may have some in his switches section of the site.