How to tie "Primary" and "Secondary" monitor spaces?

Solution 1:

I have intentionally blurred the window contents; no need to clean your glasses ;)

Disable "Displays have different spaces", that will make them move as coherent pairs. Additionally disabling 'auto rearrange based on recent' will make the numbering stick.

So - this is 2 monitors, 7 spaces. Each 'Space' covers the pair of screens & their contents. Changing Space changes both monitors simultaneously.
The 'reappearing window' top right is Activity Monitor, which is set to be on all Spaces. Everything else lives on its own Space & only appears when I switch to that Space.
The gif is a 'live' movie of me using the keyboard to call each Space in sequence, then back to the first [then the gif cycles & you see the movie title again]