This is a super old thread, but just FYI, you can't use Freesync if you have more than one monitor, EVEN IF BOTH MONITORS SUPPORT FREESYNC AND/OR ARE IDENTICAL MODELS. The monitor will work, of course, but you will not have Freesync active as long as you have more than one monitor connected. It's an inherent limitation of Xorg and cannot be fixed.
Wayland does not have this limitation, and you could use Freesync on your main monitor without having to disconnect your second monitor if you use a Wayland desktop environment session that supports Variable Refresh Rate (currently only Plasma Wayland and Sway WM). But if you're using Xorg, then you've not been using Freesync. It's impossible.
