must be pretty similar to unix based mac ln -s command. Principle is so simple direct symbolic links to the same root folder and run conversion from there.
i.e:
mkdir -p tmpfolder
cd card 1
ln -s $(find . -iname '*.M*') tmpfolder
cd card 2
ln -s $(find . -iname '*.M*') tmpfolder
Not correct paths and so on but only an example.