In the Organizer select two thumbnails and hit the F12 key to compare them. But I guess you mean in the Editor application. In Expert Mode it‘s possible to work with floating windows rather than fixed tabs but you first need to set your preferences. Go to:
Edit >> Preferences >> General (Windows)
Photoshop Elements Editor >> Preferences >> General (Mac)
Then put a checkmark in the box against “Allow Floating Documents in Expert Mode”
You can now open your two images and use the layout button at the bottom to choose various configurations or to cascade.