Presenting in PowerPoint - Issues with it taking over two screens

Now I understand what you're trying to do. When in presenter view, the presenter notes page 'takes over' a desktop, but you can have multiple desktops on your Mac. Go to System Preferences > Mission Control and tick the box for Displays have separate Spaces. Then, ensure you have at least three Spaces (desktops) by going into Mission Control (personally, I have it set to a three-finger swipe up, you can set that in System Preferences > Trackpad > More Gestures), and moving the cursor to the top of the screen, there's a (+) button at the top right. Also in System Preferences > Trackpad > More Gestures, have Swipe between full-screen apps set to something you like (I use three-finger left/right swipe).

Now, in PowerPoint you can launch your presentation, then with the mouse cursor on the presenter notes page, switch to another Space/desktop (the three-finger swipe left or right, for me). In that desktop, you can launch Zoom (or other VC app), then when you start sharing select just the specific desktop with the full-screen presentation to share. You can then have your Zoom window open in one desktop and run your presentation in the other with just the full-screen powerpoint being shared, depending on where you put your mouse cursor.

I've never experienced any issue as you describe. Are you using extended display mode? If so, you can drag any window you like to the external display. If you an issue with how display mode works in PowerPoint perhaps ask for assistance in the Microsoft Help Communities.

Is there a Swap Displays button on the presenter view (to the right of the marked button)?