Second attempt :)
Setup
RPi 4B, 2GB RAM, GPU memory set to 512MB
rpisurv 2.1.7 installed, taking rtsp streams directly from the cameras
Dahua 5442 cameras (15fps, main stream= 2688 x 1520, substream = 704 x 576)
Main stream display
1 x main stream displays reliably
Adding a second causes...