X11 With Multiple Displays

Has anyone had success attaching multiple displays to a single container? I’ve been playing with my custom version of https://github.com/balena-io/resin-electronjs as well as the stock https://github.com/balena-io-playground/x11-window-manager and I’m not sure how to get a second display to work.

I’m using an RPi 4 with the official touchscreen and an HDMI display.

I’m not too well versed in the world of x11 and Linux screens, so I’m not sure what steps to take. I’ve tried poking around the X11 config as well as tools like xrandr, but nothing I’ve done has indicated that the host or container know anything about the second monitor.