strange thing happend. :/ I had fully configured rpi4 with RO mode in fstab. I run omxplayer on dual screen and movie is mirrored on both screen properly.
Today I switched for a moment to RW mode (remove "ro" from / in fstab), restarted pi and omxplayer was not mirroring anymore - just playing movie on first HDMI. I try this a couple times (I have a image of sd card with RO mode) and this happens every time. With RO omxplayer is mirroring, with RW omxplayer playing only on first HDMI. Returning to RO mode dosn't fix that.
It seems like running rpi on RW mode change some file(s) on startup, but not sure where should I searching for those changes. :/
Hint: I've 1920x1080 display and 1440x900. With RO mode both sceens displays smaller resolution and that's ok for me. After entering into RW mode, first display has 1920x1080 and second 1440x900. Maybe this will give you some hint.