I’ve just had a quick look through the ACF code and I don’t see any way to accomplish what you’re looking to do.
Technically, in one case you have a sub repeater and in the other you don’t. There isn’t any way to trick ACF into using the repeater from somewhere else as a sub repeater. The values of a sub repeater are not stored in the database in a way that would let that happen, other than perhaps writing your own custom have_rows() function, maybe have_modules()? but that would also mean writing a custom the_row() function as well I think.
Sorry, I just don’t see any way to do it, even writing custom functions, it’s not going to be easy to do.
