Teamviewer 15.7.6 on Linux Mint 19.1
I'm connecting to a Windows PC with two monitors. Initially, the "Monitors as individual windows" box was ticked and the two monitors did indeed appear as two windows.
I unticked the box and (as expected) got a single extra wide window showing both remote monitors.
However, I don't seem to be able to go back to the individual windows mode. The checkbox seems to be ignored.