To output to multiple devices I think that the most common solution would be to use a hardware splittler. I've never heard of anyone using multiple video outputs from a single computer, and I'm not sure whether or not the macOS display mirroring would support this (one display would need to be mirrored because MixEmergency currently only supports a single output window).
As you have 2 computers, it would also be possible to send the output of MixEmergency, via NDI/network, to your second computer and then have that computer connected to a 2nd external display.
As for multiple camera input sources, you can use the Video Input in the Overlays section to load up a single video input, but you can also use other media players in MixEmergency, such as the Sample Player, to load video input sources as well. If you right-click (or control-click) on one of the Sample Player previews then it will open a context menu that allows you to assign to it a video input source.
