Okay, did a lot of "re-engineering" of my automatic ore refinery, but I think I have been able to create a setup like Lethosos suggested. All the metal dust is dumped into a single chest, with two manipulators on opposite sides. A redstone clock with an inverter on one line provides alternating signals to one manipulator then the other. each manipulator leads to it's own dedicated electric furnace, which then combine and go to a final output chest.
The split isn't perfect, (I got 30/34, then 31/33 on two separate runs of 64 items) but it is close enough to stop any one furnace from being overloaded and rejecting input which is what I wanted.
Thanks again for all the ideas. I'll be happy to post pictures, videos or the world if anyone wants.