I am running TeamViewer v 9.0.24951. Since the update to v9, the "Pop-out Monitors as individual windows" option does not function, in the following manner:
Host system:
Running v 9.0.24951
Has dual monitors
Remote system:
Running v 9.0.24951
Has dual monitors
Scenario:
Remote system connects to host. The initial screen is monitor 1.
"Pop-out Monitors as individual windows" is selected.
I switch to Monitor 2. The monitor draws within the same window.
I notice that the two windows are actuall "tabbed" within the one window, and I can toggle between them, by utilizing the tabs. This has the same effect as clicking the window-on-window button in the upper right of the control strip.
SOLUTION: Right-click one of the tabs, which represents one of the windows, and use the 'Move to new window' option. I can then move one of the windows, to the remote monitor 1, and the other window, to remote monitor 2, and thus replicate the experience of the two monitors on the host.
This differs from the prior versions' experience in that I have to make the extra step to "Move to new window". This is counter-intuitive, since the "Pop-out Monitors as individual windows" option was already selected, making one think that the pop-out would work without having to resort to a "Move to new window" option, an option I had to search for and stumble upon via the right-click method.
I.e., essentially, "Pop-out Monitors" now means "Represent each monitor as tabs, within one window". Once one wraps one's head around that, achieving multiple monitors, in multiple windows, becomes possible again.
I hope this helps.
Note that you also might experience an issue if the host's second monitor (or both) are turned off. I have to leave the host monitors on and utilize the 'Show black screen' option in order to view both monitors.