Since Manjaro is based on ArchLinux, I'll link you to the ArchWiki, also note that there is the ManjaroWiki.
- AMD Catalyst (proprietary): https://wiki.archlinux.org/index.php/AMD_Catalyst or https://wiki.manjaro.org/index.php/Catalyst
- xf86-video-ati (open source): https://wiki.archlinux.org/index.php/ATI
- Multihead (about the multi monitor setup): https://wiki.archlinux.org/index.php/Multihead. In particular I'll look into using RandR through the xrandr tool ( https://wiki.archlinux.org/index.php/Multihead#Configuration_using_xrandr ).
EDIT: if you're not familiar with the command line, or don't want to use it, you can use arandr, a visual front-end to xrandr.
Have you tried setting the settings with Arandr? I had some issues with monitors not correctly retaining their orientation. (All Ubuntu based though). You can save the configuration as a shellscript and execute it at startup. Works good for me so far. Although I do not have driver troubles since I used debian/Ubuntu.
Did you fix it?