Thanks for the helpful details for your question.
You cannot import one project file (.prel) into another (.prel).
Since you have Windows 7 64 bit, there are two approaches to combining the contents of two projects into one
1. You can export the contents of project 1 to a file saved to the computer hard drive. And, then import that export into project 2 for final export.
The project settings and export settings should be consistent project to project. We can go into specifics based on further details of your projects' source media and export goals. If you wanted, you could take the export from project 1 and 2 and import those exports into a new 3rd project.
2. You can use ClipMate (Windows Only software) in conjunction with Premiere Elements to do a copy/paste insert of Timeline content between Premiere Elements projects. Please refer to the following for full details
Please review and consider, and then let us know what supplemental information you may need.