OK, folks, had a great chat with @arbomambo .. As I suspected, my misunderstanding came from false assumptions.
I had been thinking all along (based on a typical home layout) that a bus was two wires, one + and one -. But a bus can be four wires (two + and two -) just as easily. And in that case, just reverse the connections on the second + and - pair for desired polarity. This actually makes perfect sense now. Sorry for being so thick-headed!
Thanks for translating that into English. There's a huge gulf of disconnect, particularly with respect to the terminology, between those that understand how this stuff works, and those of us who don't.
