While there is usually 1 CRTC frame per screen frame, the screen frame can be divided into multiple CRTC frames of varying proportions.
This is useful for defining different scrolling zones in your screen frame. For example, it was used for hardware parallax scrolling in the game [https://www.cpc-power.com/index.php?page=detail&num=1307 The Living Daylights], released in 1987. And this is basically the bread and butter of innumerable CPC demos.
Some rules of thumb [https://www.cpcwiki.eu/forum/programming/advice-on-split-screen-code/msg242186/ Source]: