Welcome to the forum.
Unfortunately, PrE will not allow two Projects to be open at the same time. It will also not allow multiple instances of PrE to be open.
The workaround, to accomplish what you describe, is to use a ClipBoard Extender, such as ClipMate, which will allow one to Copy a Timeline, or parts of it, from one Project, then Close that one, to Open another one, and Paste the material, that was Copied. Many users employee ClipMate, or similar, to Copy/Paste material from one Project, to another.