MD and DO application services do not communicate with each other, so neither will know you're applying via the other application service.
I read on SDN that it is sometime in March that the list of your MD acceptances is available to other schools. It's correct that you can hold multiple acceptances until May 15, by paying a refundable deposit. After May 15, you can stay on any waitlist you are on, even if you hold an acceptance. If you are accepted off a waitlist by a school you prefer, you can drop the previous acceptance, but you will not get your $100 deposit back.
I don't know if DO school acceptance information is available to other DO schools or not.