Hover over each screen you want to move and check the circle on the screens you want to copy to another project.
Scroll to the bottom, click the more button ("..."), click Copy to prototype, and select the appropriate prototype.
Copying screens to another prototype does not copy over hotspots or comments.
Ensure the file name for each screen is unique across prototypes, otherwise you may overwrite the existing screen when copying. For example, if you copy a screen from Prototype A and that screen shares the same file name as another screen in Prototype B, the screen from A will overwrite the screen in B.