1. You can add 2 configurations:

cam1:

runOnInit: bash -c 'rpicam-vid -t 0 --camera 0 --nopreview --codec yuv420 --width 1280 --height 720 --inline --listen -o - | ffmpeg -f rawvideo -pix_fmt yuv420p -s:v 1280x720 -i /dev/stdin -c:v libx264 -preset ultrafast -tune zerolatency -f rtsp rtsp://localhost:$RTSP_PORT/$MTX_PATH'

runOnInitRestart: yes

cam2:

runOnInit: bash -c 'rpicam-vid -t 0 --camera 1 --nopreview --codec yuv420 --width 1280 --height 720 --inline --listen -o - | ffmpeg -f rawvideo -pix_fmt yuv420p -s:v 1280x720 -i /dev/stdin -c:v libx264 -preset ultrafast -tune zerolatency -f rtsp rtsp://localhost:$RTSP_PORT/$MTX_PATH'

runOnInitRestart: yes

and access them via link

http://IP_ADDRESS:PORT/cam1

http://IP_ADDRESS:PORT/cam2

2. There should be a software to stream. It could be mediamtx, gstreamer, ffmpeg, but I don't know for sure who else supports it.

Why don't use mediamtx? What are the benefits of shifting to another software?