Hmm all I know is that there might be solutions in this few links, first one is from this same forum, a bit older but I think it could help: Multi monitors panel layouts and background switcher
Second one is from last year and it has to do with specifically configuring frames for multiple outputs in Ubuntu and since this is still Ubuntu at core should work on MATE hopefully: How to configure Ubuntu Frame for multiple outputs - Docs - Ubuntu Community Hub
I also think this might work but do save/prinscreen your setup if it does not so you can reconfigure it back: multiple monitors - How do I configure xorg.conf for two X screens on one Nvidia card? - Ask Ubuntu
Also this might be and interesting read for your trouble: https://www.phoronix.com/news/X.Org-AsyncFlipSecondaries
EDIT: from the main Ubuntu wiki: X/Config/Resolution - Ubuntu Wiki - guide to setup monitors with much needed detailed steps