unfortunately there is no way to do this in the med school application process. you can apply as a couple ('couples match') when you interview for residencies and hospitals will attempt to keep the both of you in the same geographical area, if not the same hospital, but we're on our own for med schools.
i went through this two years ago. my long-time significant other and i both applied to the same schools and crossed our fingers that we would wind up close to each other. we primarily stuck to the few schools that were in the area we live in (we live in a big city). well, he was accepted pretty much everywhere we applied while i got in nowhere!
it wound up working out, for the time being, because he ultimately chose a school that is just down the road from our apartment and i started grad school at a university that is nearby too, so we didn't have to move.
i'm reapplying now and i may ultimately wind up having to make a tough decision. i'm applying to way more schools this time around than the first time simply because i have to maximize my chances, most of which are far away, and it may very well turn out that i will go to school far from him if i happen to get into one of my reach schools and simply can't turn the opportunity down or i simply don't get in anywhere close to home--and since he's in med school right now, he's stuck in the same spot for the next few years and can't move away.
anyway, i don't mean to take the topic away from you, but just to let you know, you're not the only one in this boat. i really, really wouldn't send a letter in with your applications stating that you want to go to school together. i've never heard of this working out. it's hard enough to get into med school and not to sound harsh, but i don't think schools will be concerned enough about your relationship to have to go the extra mile to purposely admit you both. just apply to the same schools and cross your fingers--that's my best advice. i would say that you stand the best chance of going to school together at the state schools in your own state, since you both have state residency working in your favor (i'm assuming that you both are from the same state). at schools with no residency requirements or little preference for state residents, all bets are off.
good luck to you.