i have no experiance with multiple monitors but i do know that very few modern video cards work well with free software you can get a good one that will work with trisquel from thinkpenguin though: https://www.thinkpenguin.com/gnu-linux/geforce-8400gs-1gb-pci-express-20-video-card-gnulinux-full-low-profile-brackets
a topic was posted about video card that work with free software here: https://trisquel.info/en/forum/best-gpu-works-free-software
