Despite what the Object Model docs seem to imply, PrinterStyleSheetPath cannot contain multiple paths (as of 2004), so I wouldn't expect your first example to work as intended.
Your second example works fine for me if I substitute an actual path on my system in place of "M:path." Is M:path a lisp variable? If so, the problem is that you placed it in quotes.
I tried this in AutoCAD 2000i and 2004. What version of AutoCAD are you using?