Tube and Solid state

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.   

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

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.

No that's not happening. 

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

^^ this ^^ 

details truly matter

You're just tooooo picky.. :-)

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

"...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. . .