Page 85 on the other forum has several pics from a user that tested his previous external display setup on a M8X.
https://yamahamusicians.com/forum/viewtopic.php?t=20021&start=1275

The remote touch screen still works with my original equipment.

Guess what? The second screen is up on the remote display as well. How cool is that?

Once I figured out how to make both touchscreens show at the same time. Something about the timing if the connection.

One trick is you have to use the double button press of "Utility" + "DAW Remote" in order to active your remote touchscreen for calibration.

I'm using the second provided USB (I suspected Yamaha added that for this option) which is different from my video I went straight from the M using both cables plus I have the USB hub for additional connections.

If I'm not mistaken for previous models Yamaha said using an external display was NOT supported. That likely applies to the M8x unless Yamaha says otherwise. That was one of the questions I ask Yamaha support about but although they answered my other questions they didn't answer the one about using external displays.