@Lvetula Sorry about that! The original post was from 4 years ago, and our support sites have changed since then.
Here is an updated guide to managing multiple monitors through GoToMyPC.
What I might also suggest is that if you don't like the automatic scrolling between monitors when your mouse scrolls onto them, you can change your viewpoint to match just your primary monitor, and use the arrows on the sides of the G2P Client Viewer to toggle between different, active monitors.
I think I just solved this. On your Mac go to System Preferences>Mission Control and uncheck "Displays have separate spaces" and Restart. Now when you launch GoToMyPc make sure it it's on "show primary monitor and scroll." Stretch the window across both monitors and you should see both of your host monitors. I had to play around a bit to get the "seams" to line up, but it works.