The USBC connections on the back of the Mainstream should be thought of as a USB hub.
The Mainstream can carry the video of two devices (HDMI video and USB-C video) to computer but it is up to the video capture or stream software (like OBS) to catch the signals and switch from signal to signal.
The HDMI device will show as the MAINSTREAM but the USB-C devices will show up as themselves as if you are plugged into a USB hub.