It should be the capability of the monitor.  I have two connected to my desktop, one 1024x768 and one 1920x1280 and they're both running native resolution

Depends on the OS. For instance, Windows 10 usually defaults to duplicating the existing display, which forces both displays to be the same resolution regardless and I presume, limited by the highest capability of the lowest resolution monitor. If they are different native resolutions, you can only mix different resolutions by selecting "Extend my displays" in the Display Settings.

Ta, all.

what are you trying to accomplish?

ultimately the answer is you determine the resolution of it as you can set/specify it in the OS settings. Though you would usually use the native resolution of the minitor for the best picture.

     >what are you trying to accomplish?<

I am receiving a video stream from a colleague's overseas site (they don't release the file itself), which I wish to record.  I can achieve this by HDMI cable out of my desktop and into another computer (the second monitor in effect).

To record the stream, I have used a USB video capture dongle on the second computer (passes through an HDMI-to-USB device too) … does the job nicely but the picture is 'squashed' a touch.  The first time I tried this approach I just fed the signal back into the desktop, with the result of a perfect picture, but tinny/echoing sound (presumably feedback).

Using the second computer, the sound is perfect, but as I say, a squashed in picture (clicking full-screen doesn't fix this).

Is there some copy protection carryon that stops you from screen recording on the PC playing it?

     >Is there some copy protection carryon that stops you from screen recording on the PC playing it?<

I don't think so, just that I could not find a half decent program to do that without spending many $$$$ (and need sound as well as picture).

PS  Just realise Game Bar is available … I wonder if that would work?

@richms … thanks for that, I'll give OBS a whirl.