How do I connect a dual monitor VNC client to a headless VNC server?

Question

Right now I use RealVNC server, and connect to a corporate RHEL server for my builds. These machines are headless, so there's nothing in the /etc/X11/xorg.conf*

I can successfully connect and use the -xrandr flag to swap resolutions on the fly, but my question is not that.

I want to go "full screen" on the client side, and have it span 2 monitors on the client side. The catch is that the monitors are not identical resolutions. I cannot simply double the -xrandr resolution and then set "useallmonitors=true".

I have sudo access on the server machine, so I can make any changes needed.