It's not hard, the "precise distinction of law," is "unlock this with your finger, whichever one does it." I don't know what complicated back and forth you're imagining, but it's never occurred in any case that I've heard of.
they would effectively be asking you the question "which finger did you use to lock this phone" to which you may plead the 5th.
We already covered this in the link above: the 5th Amendment covers passcodes, not fingerprints.