I would really like to keep the system in full stereo. There has to be a way to do it with wiring. I'm just not sure exactly what needs to be done.
If you need to maintain stereo, that will use up two of your six available channels for the horns for starters. That leaves four channels left for 12 12" cones. I'd probably put one cabinet on one side of each remaining amp, and two on the other side. That would give you a nominal load of 2Ω on one side of each remaining amp, and 4Ω on the other. The amp running the highs would be loaded to 2.8Ω (nominally) on each channel, although presumably the high channels won't be running nearly as hard as the lows (in terms of average power).
It doesn't matter that the load is not the same on the two channels of each amp. The gain remains the same until the amp clips or goes into current limit. The channels that are loaded to 2Ω will go into current limit first, but things will be so loud by then, I doubt you'll notice a slight image shift towards the other cabinets.
A better solution would be to add another amplifier. Split them up into two racks, with 3 channels for lows and one for highs. That also keeps your speaker cables short which keeps the power loss and effective damping factor (as much as that may or may not be important) as low as possible. Even if the amps are all in one rack, run a home run to each speaker from the rack. Jumpering at the speakers doubles the load on that long run, and you lose power and damping factor.