I like a lot of the Team Viewer capabilities, but it's hard to believe you've made it to Ver 12 and still do not have dual monitor support.
With the native Windows Remote Desktop, the remote session takes on the attributes of the host system (if you want it to) regardless of how manay monitors the remote system has (even if the remote system is headless, with no monitors). This is awesome, and how a lot of us want to log into our remote systems. When I have 2 monitors on my host, I want to be able to view my remote using both monitors, as if I'm sitting at that remote system and it has 2 monitors. Just like Windows Remote Desktop can do:
When logging into a remote system, and choosing to use both monitors, I get a full layout using both monitors. And, if I go full-screen with the remote desktop, it is just like I'm sitting there at the remote system with 2 monitors - no boarder, just a perfect dual monitor experience with full 3840x1080 resolution (1920x1080 extended).
Then...there's team viewer. Horrible. Team Viewer logged into the same remote computer. No ability to take advantage of 2 screens. Huh??
It's 2017 folks. More of us are using dual monitors than ever before. We expect a premium product like Team Viewer to give us a premium experience. High time for you guys to catch up to what Remote Desktop has been able to do for 15 years or more!
If I'm missing something in TV - a setting somewhere that allows this, just let me know. But based on dozens and dozens of blogs and posts I've read about this issue, I'm going to guess you guys just have still not figured this out yet. Time to!!