All I need is basically a first order low pass (satellites are full range) but was having trouble figuring how do that without commoning the L/R channels for the satellites and effectively making everything mono.

I remember the Sequerra speakers. I think that was the case (dual voice coils). I was very impressed by them at the time.

picture from http://www.zenn.com.sg/Sequerra_spkrs.JPG

The more I thought about it, would it work to build two crossovers and then common the signal line of the load side of each xover (on the low fequency output) to the shaker?

I can put in a summing circuit easy enough, but most of the circuits I have looked at combine summing with an active crossover, which increases the complexity quite a bit.

What I may try is using a passive xover with a summing circuit either before or after the xover.

On a 4 ohm load, what resistor value (if any) would be appropriate to but in series with the capacitor on the low frequency side (second order xover)? I'll be crossing over at about 100Hz, IIRC, so the cap is about 500uf.

The plate amps have to sum the signals and cross them over at some point. A plate amp has a seperate amp for the sub, but can still accept amplified full range signals. The plate amp will need to filter, sum, and amplify. I was simply trying to find a way to remove the amplify piece.