Does anyone know if I can have more than one subtitle in the carousel cards?

You can't 🙂 Just a title and a subtitle. You could add an image with some text, and use that as a second subtitle, but it won't look good 🙂 Same goes for using the Action Buttons as a subtitle.

@fresh-fireman-491 Thank you, I was afraid of it

Why do you need 1 more?