Cecil, W5DXP wrote:
"I wonder if two separate amps could maintain coherency under all
conditions of supply voltage, temperature, humidity, etc."

If reasonable control applies, it works.

I built a rat-race, hybrid-ring, diplexer to parallel two identical
shortwave broadcast transmitters. They were fed from the same RF
oscillator and from the same audio source. Both were adjusted for nearly
100% modulation. The pair produced nearly 100 KW, fully modulated into
the same antenna. It worked fine. No jerking around and little
unbalanced energy dissipated into the dummy load. This was no sweat for
the antennas which worked with our 100 KW transmitters every day. This
is what you need to do when frequency shortages become acute.

The votages in the transmitters were regulated by Sola transformers
which had been shipped to us as 60 Hz transformers but reresonated on
site for 50 Hz which was our European supply frequency.

Best regards, Richard Harrison, KB5WZI