Nobody has addressed what happens if the OP is accepted to both schools before withdrawing from one.
Obviously, there is theoretically something to gain from telling 2 different schools "you are my first choice and I will attend your school if I am accepted." If there wasn't something to gain, you would just tell them the truth: "you are one of my top choices and I will attend your school if you are the first one to accept me."
Well, as people on this thread have speculated, most likely, nothing will happen if she's accepted to both schools simultaneously and has to withdraw from one, but as I said earlier, s/he would then be forced into a position of going back on his/her word to one of those schools. It is definitely a moral risk, albeit one without any physical consequences, but the OP needs to decide for him/herself whether s/he wants to take that risk. And in the interests of full honesty (and as I mentioned before), the OP really shouldn't tell both schools that they're both his/her top choices, because that would be lying. S/he could simply say, "I'd be thrilled to be part of your class of 2011, and I would attend your school if accepted."
The latter statement you propose would definitely alleviate any sense of moral guilt, should the OP find him/herself in the position of having been accepted to both schools without having had the chance to withdraw from one. But I agree with you in that the latter statement is less potent than the first and can even come off as a bit obnoxious, placing undue pressure on each of the schools to "act fast, the OP's deal won't be around forever." However, if the scenario of both schools granting their acceptance simultaneously doesn't come to pass, then sending two LOIs with "I'd be thrilled to be part of your class of 2011, and I would attend your school if accepted," and then following through with withdrawing from the other waitlist upon receiving an acceptance doesn't constitute lying. S/he is still well within his/her rights to withdraw from the other school's waitlist PRIOR to the school granting that acceptance.
I'm certainly not saying that this is a good or honorable idea to do this, but ultimately only the OP can decide whether s/he wants to take the moral risk of sending two LOIs and receiving 2 acceptances simultaneously. It is a somewhat unlikely risk, but a risk nonetheless. We can't make the OP's decision for him/her.
I personally couldn't do it, even when I was agonizing over two schools. I thought very long and hard about it, and ultimately, I sent the LOI to only one school and a letter of interest to the other. However, there's always a risk of not being able to follow through, even in sending only one LOI (e.g., what if the school gives terrible financial aid, what if the applicant has a last-minute change of heart, what if a major family situation occurs). I fully intend to honor my LOI, but I'm also expecting a reasonable need-based financial aid package that takes into account my and my family's demonstrated need. Luckily the school is known for generous need-based aid, so I felt I could make that LOI commitment safely. That's why it's important to consider LOIs carefully. The schools themselves probably don't really care if you back out of one, but there's something to be said for helping to make the entire med school application process a more honest one, rather than a process in which everything is just part of "playing the game" of med school admissions.
