I'm using a chromebook connected to an external monitor for my job and we use Horizon for our workspace. The issue is I'm not able to get Horizon pulled up on both screens. I can maximize it on the monitor, but it'll only display there. If I maximize it on the chromebook, itll pull up on both screens but it'll be a blank white screen on both. This issue has affected a few people i work with, but some have no issue. Any ideas?
From your description, I think the Horizon Client from Google play store was used. It is expected behavior that the device could enter multi-monitor mode only on maximizing the session window in the device screen but not the external display. The behavior may be changed in the next release.
For the issue showing white content for multi-monitor mode. I think it is the not enough VRAM issue for the VM. Setting more VRAM for the VM should help, or you could try to reduce the client resolution, by setting "1/2 scaling" in the Horizon client settings -> display -> resolution.