How can I open more than one instance of Microsoft Project?

Short answer, there is no easy way to do it. Project is designed to only allow one instance running at a time. There are a couple of convoluted solutions out there dealing with running it as a separate user, which work with only limited success, if at all.

Here's a discussion that discusses a couple of possible workarounds. But basically your options are to run a second copy under a VM/terminal service or as another user id locally (which may or may not work).

You might also investigate alternative software that might offer that feature and be compatible (to varying degrees) with MS Project files.

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".

---Select "OK"

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.

from:

https://microsoftproject.uservoice.com/forums/218133-microsoft-project/suggestions/13586853-open-multiple-instances-of-project-for-side-by-sid