Activity Monitor - How to view multiple panels at the same time
Solution 1:
You can choose your own set of columns to be displayed on any particular tab by right-clicking on the column headers (or View → Columns) and checking columns to be displayed. For example, this lets you show the Memory column while on the GPU tab.
If you really want to display two different tabs using two windows, you can launch a second instance of Activity Monitor with open -na "Activity Monitor".