Here's what resolved this for me: plug in the lower hz monitor first. I have two external monitors I use with my M2 Macbook Pro 16" (along with the Macbook display itself): 60 hz 4k monitor 240 hz 4k monitor. Both are plugged into USB-C/Thunderbolt 4. TB4 only has so much bandwidth, so it is unable to run both external 4k monitors at their highest 60/240 hz refresh rates. However, there is enough banwidth to run them at 60 hz and 120 hz. The problem is, if I first connect the 240 hz display, MacOS allocated bandwidth based on available resolutions and refresh rates, the max being 4k/240 hz. This consumes all available bandwidth, so plugging in a second monitor means the second monitor doesn't get any bandwidth, and thus no display signal. MacOS does not re-negotiate available resolutions/refresh rates for displays with already-established connections. The solution? First plug in the 4k/60 hz monitor, then the 4k/240 hz monitor. The second monitor will only get enough bandwidth to support up to 4k/120 hz, but now at least both monitors work.