Hi Daniel,
If I’m understanding you correctly, someone has implemented a plugin that does what you’re asking for. It’s called ACF Reusable Field Group: https://github.com/mvpdesign/acf-reusable-field-group
Let’s say you have 2 field groups you want to choose from: FG1 and FG2
Create a 3rd field group (let’s call it Repeater Group) that’s a repeater with a Reusable Field Group field in it. Add the Repeater Group to your pages.
Then, when filling out the page, you would add a row to the Repeater Group Field, select FG1 and fill it out. Add a new row, select FG2, and fill it out. Just watch out for ACF Inception, I think it might be possible to select the Repeater Group itself and create a field within a field within a field. 🙂
