Assuming they are currently all in parallel, all you need to do is split the live at the mid-point and run both lives back to the switch. On the diagram above's wires to the lamps, top and bottom wires are the two lives after splitting them and the middle wire is the existing neutral. Same number and type of bulbs each side of the split of course.
This is really interesting to me. I had to concentrate for a long time before being able to understand it enough to ask questions.
I already have a spare unused wire running through the first half of the conduit, because there used to be 2 light switches, 1 for each side of the room, and I combined them into 1 switch a few years ago.
Forgive me for breaking this down and simplifying it, but could you please check over my reasoning here?
-When the lights are turned on full power, the existing neutral acts like a regular neutral wire. So we have 2 hot wires (one for each side of the room) and 1 neutral wire.
-When the lights are turned to half power, the neutral wire is disconnected from the box, and doesn't act like a regular neutral anymore. Instead, the neutral acts as a connector to create the series pairs. One of the previously-hot wires now acts as the new neutral, completing the circuit back at the box.
Was I far off?
Thanks for the info about the halogens. I had never heard that before, and it was good to learn. I'm actually planning on using regular incandescents here. The reason I had mentioned halogens earlier in thread is because I got the "rectifier diode as a dimmer" idea from a halogen fixture I had opened up to repair (it was this fixture here, which used a 1N4007 diode for the "Low" setting:
http://www.amazon.com/gp/product/B0006SUZJM). The bulbs on those halogen fixtures seem to have good life spans to me, even though I'm running them on Low most of the time.
I'm also still considering the idea to use a series dimmer module inside the box. But I'm really enjoying learning all these useful techniques, and I would like to understand them before I make a decision.