How to get dual display output on RPi 4B?

Idea is to have two display devices connected to two HDMI ports of Ground RPi 4B and have OSD/video on both of these displays. I do not expect these displays to be able to run different resolutions, so basically mirroring is needed.

With default openhd image configuration, first HDMI port shows OSD/video and second HDMI port shows console (after pressing alt+12).
There is no x-server, so there is no xrandr tool to do mirroring at this level. So I thought maybe it would be possible to do something like framebuffer copying, but then I found that by default there is only one framebuffer device (/dev/fb0) even when two displays are connected.
I am lacking of knowledge on how that Linux video system works under the hood, so I am looking for suggestions on how this could be achieved in the right way and what should I google for:)