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. . .

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

You're just tooooo picky.. :-)

You don’t have two amps. You have an integrated amp and a power amp.

^^ this ^^ 

details truly matter

No that's not happening. 

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.

Most of the current Mac gear (C and MX series) has an A/B along with a fixed output for a tape monitor or pass through. You can split the outputs like spatialking suggested also.. I use both methods.

Regards

Should not be a problem.   Use a RCA female to RCA Y male splitter.  I do this a lot since I have multiple subwoofers.   Most preamps should handle the load, since amps have high input impedances and preamps can usually drive that without a problem.  If the input impedance is less than 10,000 Ohms on one of them, you might have a low frequency roll off of some sort, but even that is unlikely.