I was looking at using one Raspberry Pi4 to do multiple tasks in GB3KM.
The concept is using the USBTV0007 Easycap to take in Composite video and output this on HDMI with the correct aspect ratio, this does a better job than the Composite>HDMI converters off ebay etc.
The original idea was to use two easycaps on one pi4 with two instances of mplayer, unfortunately i could not find any means to select the display for video out.
Fortunately there is an option for this in vlc
To display an easycap input on HDMI1 port simultaneous with the Ryde receiver on HDMI0 you can use this command, simples!
vlc v4l2:///dev/video0:width=720:height=576 --mmal-display=hdmi-2
One unfortunate problem is that it does not always show the noise like it did with mplayer when i last tested it, maybe a v4l2 update broke that...