How to synchronize PIC PWM clocks?
Read about disciplined oscillators.

Some MCUs allow to lock the internal clock to an external signal (say 10KHz),  saving you the circuit to upscale that clock.

You would need to distribute that signal (would RF be acceptable?)

If you don't want to distribute your own synchronization signal, GPS might be an alternative.

GPS distributes a  1PPS signal. You could buy a GPSDO or make your own through PLL or FLL circuits and a GPS module.