Specifying FatFileSystem for ifstream

I am using ifstream to read a text file from SD card.

I also have a flash chip.

Both are open as FatFileSystems at the same time.

When I use ifstream, I cannot specify which FatFileSystem to use. It seems to use the last FatFileSystem opened, but that is not always what I want.

The ifstream class has a private Private Member Function which allows the FatFileSystem to be specified, but I can't figure out how to invoke that via the available Public Functions:

bool open (FatFileSystem *fs, const char *path, oflag_t oflag)