Tube and Solid state

"...The thought was to run six Cornwalls. Four from the Yamaha and two from the McIntosh..."

No problem. Use a Y interconnect out of the Yamaha Pre-out. One pair of cables back into the Yamaha and one to the Mac. You may have a mismatch in volume you might have to deal with but other than that you are GTG. . .

You don’t have two amps. You have an integrated amp and a power amp. The proper way to do this in your application (driving one pair of speakers with two amps) is to use the integrated amp as a preamp. So the path is: Yamaha preamp out (split) to a crossover. Crossover to each amp. Amp to speaker split inputs (one highs one lows) Is that what you are trying to do? .
If you are driving two pairs of speakers with two amps, you can toss the crossover.