More info: The 2 cameras are a canon 70D and a canon 6D. They are each connected via hdmi to an ultrastudio mini recorder and then by thunderbolt to a macbook pro.
If the cameras don’t support using HDMI and the camera monitors at the same time, there are basically two options left for you:
There might be an alternative firmware for the Canon cameras available on the internet that might support showing the video on the monitors and the HDMI output at the same time. However, we can’t provide any assistance on this and it might violate the warranty of the cameras.
You can acquire an external HDMI video monitor like the Blackmagic Video Assist.