Android Studio Multi-Windows for One Project

If you click and drag a file's tab out of your current window (over onto the desktop, for example), the file will open in a new window. However, this window doesn't seem to have the whole package explorer, console etc.

Although, if you click on the gear on any of the modules, you can uncheck 'pinned' mode and you'll be free to move them around your two monitors just like the window you created above.

Window -> Editor Tabs -> Split Vertically

if you don't use files tabs

Right click on tab of the file and click "Split vertical / Horizontal"

After splitting, you can just drag one of the tab titles to the second monitor - it will open up a new window. You can then edit the same file in two separate windows