"mixed" or "merged" submodules

Hello,

here  https://stackoverflow.com/q/72770397/1876484

I asked this question:

I'm aware of git submodules which dwell each in its own separate directory.

1. But is there such thing as "mixed" submodules whose content is
"merged" together?

For instance:

- Submodule1 (path ./), consist of files `a.txt`, `b.txt` and
directory `C` with the file `1.txt`
- Submodule2 (path ./), consist of files `x.txt`, `y.txt` and
directory `C` with the file `2.txt`
- Resulting "mixed" repo of both submodules: files `a.txt`, `b.txt`,
`x.txt`, `y.txt` and directory `C` with the files `1.txt`, `2.txt`

2. If it is not implemented in git - is there a workaround to achieve this?

Here my use case:

Both submodules - independent libraries (collection of books as plain text files), which have same structure (directories = book categories). I want to present the combined parent git repository as full collection of books, while both projects evolve independently and do not overlap (in terms of file names = books).

I got a very detailed and informative answer. My question now - do you see any other practical use cases for such a feature? Would such a more general case of submodules be a good feature in git or not?

Thank you!