Chainblu is correct on both accounts. according to dish network (both internal only) and EKB documents, the DPPlus twin is supposed to work with 2 legacy receivers, and is supposed to switch off the third input and be able to be powered in this manner. I have tried this on an install, and it works fine (actually both legacy receivers saw the third input as well) unless both receivers are tuned to odd transponders. As soon as I did that, they would both loose signal. This makes sense, since the LNB really has no way to get 18 Volts when both receivers are only sending it 13V to request odd transponders from the switch/LNB.
My recommendation is to either have at least one DP/DPPlus receiver connected, or two legacy receivers, with at least one having a DP Adapter on it (The DP Adapter is more than capable of fully powering a DPPlus LNB, even with a wing dish if necessary).
Good Luck