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.