Clicking the button to the left the Panadapter close button will expand it back to its normal state.
When you have multiple Panadapters open, this button allows you to maximize a specific Panadapter/Waterfall to the top area of SmartSDR. When you do this, a gray bar with 4 dots will appear underneath-- you can drag this bar to resize this top area to your liking.
You can see this gray bar in the image that you posted underneath the second Panadapter. In your case you have both of your Panadapters in the "Maximized" state at the top of SSDR. For most PC monitors, I find it to be most effective to have only one large Panadapter in the Maximized state, while having several smaller ones underneath.