show same window on both screens?

MacOS Monterey

Have two screens beside each other. One as "main", the other as "extended". No problems grabbing and moving windows and stuff moving between them. Have also set e.g. Photoshop to display a variety of those extra small menus on one screen and the actual image on the other.

But Apple Garageband doesn't allow me to "pull out" stuff from its work area and move to the other window. Hence, the screen displaying Garageband is crowded and each part making up for the work area becomes tiny.

But since I have the screens side by side, I can just make the Garageband work area wider so it covers both of them. Well, in theory.

Because the work space "disappears" in either of the screens, so I'll only see half of the now much wider work space.

How can I make both screens actually show what's on them, even they each show half of Garageband?

(Aaarghhh, I don't think I managed to explain the issue very well. Please ask for clarifications.