Fix multiple Main Window issue - broke around June 2017
Until recently, you could have two "Main Windows" open (File > New > Main Window) and use one window for Mail and the other for Calendar. This was great for me on my dual monitors, as I could have one window on each monitor.
This ability broke in the last couple of versions on the "Slow" development track, I think around the time of the June 2017 Slow Build release. The second window will still open, but you lose function and/or display information. For example, you can click in an email thread but the mail display will not update, or one of the two windows will be empty/blank. This is a real bummer for me.
Rault Kehlor commented
This seems to be working for me in Outlook 16.10 (updated Feb. 12, 2018, normal release builds). Thanks for the tip! I've been wanting separate windows for a while.
Both Mail and Calendar windows open and are functional, without any glitches like you described.