I got (free) samples of the Stereo hat from Lee at Arducam.
Yes, you will get 1 image consisting of left and right camera images.
In case the cameras are both Raspberry v1 or v2 cameras, you get the exact resolution determined by the mode.
That means that each camera contributes only "width/2 × height" to the "width × height" frame for modes 1-7.
Reason is that Raspberry Pi has no idea that there are two cameras, it knows only of one.
These cameras allow to use raspivid and raspistill with Stereo camera hat:
https://www.raspberrypi.org/documentati ... /README.md
In case the cameras are Arducam models (0.3/1/2MP monochrome global shutter or 13/16/18MP color rolling shutter cameras) the result will be "2*width × height" frames. In software you can split the frame in left and half side and display on two monitors. You can connect DPI/DSI, HDMO0, HDMI1 and composite to Pi4B. What I heard was that only two of the 4 monitors can work at a given time.
"raspivid" allows to specify "--dispnum X" and decide on which display the preview window should appear (not shipped yet, if really needed can be made available with rpi-update).
Here is sample of (picamera) camera previews of one v2 camera on two displays (DPI and HDMI0):
https://www.raspberrypi.org/forums/view ... 7#p1582997
Here is example Arducam monochrome global shutter cameras with Stereo hat.
2*1280 × 800 = 2560 × 800 frame:
https://www.raspberrypi.org/forums/view ... 2#p1569676