we have managed to get our offline version to work with two monitors. 2xfullHD seems okay, but we wanted 2x 2560x1080 (21:9) displays, where we tried to play two 1280x1080 videos, and it started to tear. It looks like a sync issue between the screens. Have you ever seen this, and is there a solution?
I’ve seen this and it looks like there’s no way to fix this at the moment: https://www.raspberrypi.org/forums/viewtopic.php?p=1583405#p1583405
We’ll have to wait for a firmware that fixes this issue.
What happens if you configure the Pi for dual 1280x1080 HDMI output? The visual result should be almost identical (the display does the scaling vs the Pi composer does the scaling). Can you try that?