I found a solution on another forum that works on 2016 and 2010 :
Edward Medri · WORK AROUND (MS Project 2016)
1) Open one project file with MS Project (double click on the *.MPP file)
2) Open second project file with Internet Explorer (right click on the *.MPP file and Open With)
--- A Dialog Box Message will pop up with following text,
"What do you want to do with filename.mpp?"
--- Select "Open".
--- Then the Internet Explorer window will have no content (blank page). It is OK.
--- Switch back to MS Project to view a new Dialog Box Message with the following text,
"Project Cannot Open the File".
VOILA ! !
You now have two separate windows, with two different filenames projects that are each editable and savable.
Note: When you save with new filename, it will create and save BUT does not display/update in the browser.