Don't move your windows from one display to the other.
Move all windows from your main display to your secondary one, and the other way around, at once.
Swap them quickly using a keyboard shortcut or even easier with a systemwide menu icon.
Preferences
Customize Swapp to fit your way of working.
Open at login: launch Swapp every time you log in to your Mac.
Show in system wide menu bar: uncheck to hide the menu extra icon.
Click action: choose your most frequent kind of swap and run it with a click.
Mouse button: left or right click on menu extra icon. The other one will show the menu.
Instant swap: short of resources? Disable the swap animation.
Disable soudns: mute all sounds.