I did a master's in bioethics before starting medical school. it was a great opportunity for me to integrate my interests in philosophy and medicine before being throwin into the science-heavy medical school curriculum. i am now an MSII and don't use any of my bioethics knowledge...as yet.
i did the master's at a different school than my medical school, so i can't comment on doing a joint degree. however, i did know some students who did joint degree. most of them did it after their four years of med school. that's probably the best way, in my opinion.
i did the masters at UPENN so i know that you had to be admitted to the MD before adding the MA in a joint degree (or any other conjoint degree: MBA, JD, etc.). Some tried the MA first to bolster their application to MD at PENN, but as far as i can tell, it didn't help to the extent they had hoped.
hope this helped.