This is my take and response to what you have asked.
You cannot import a Premiere Elements project file (project.prel) into another.
And, whatever is on the Timeline of a project needs to be exported as a video file OR be copyed/pasted between two projects (project.prel files).
From what you wrote, you do not want to "export both as video files" as Hunt and Ann Bens have detailed in a recent thread.
As for copy/paste, the only way that works for me is achieved with 3rd party software. I have used the program ClipMate which is a Windows Only software. And my how to use it is described in my blog post
Please note that with Premiere Elements 11 Windows and ClipMate, there is a problem in the copy/paste of transitions. All else seems to be OK as described in the blog post cited.
Please review the above and let us know if I have targeted your question.