Here is what you are looking for:
Open the first Project Plan through MS Project then Open the second Project Plan and then Select from the Menu “Windows” then select “Arrange All” and there are the two project plan shown within the same window.
You can utilize your dual screen setup to view 2 project files.
First, open MS Project and click on the Maximize button (top right of Project) so that MS Project is not maximized.
Second, now you can grab the corner of the project window and drag/stretch this to fit across both your screens.
Third, open your 2 project files. You will then also need to move your files and stretch them to fit a single monitor. The idea is that one file will be the size of your first screen, the second file will be moved and then stretched to fit across your second screen.