What version of Premiere Elements and what computer operating system?
The general answer to your question regarding "Merging Projects" is....
You cannot import one project file (.prel) into another project file (.prel).
But, you can xport the Timeline content from project 1 to a file saved to the computer hard drive, and then import that file from the
hard drive save location in project 2. Keep the project and export settings consistent from project to project.
If you have Premiere Elements on a Windows computer, you might consider using ClipMate in association with Premiere Elements for a copy/paste insert between projects.
Please review, consider, and give more information so that we can give you more specific information.