Be sure to change all the "Darthmouth's" in your application to "Dartmouth". Microsoft Word could probably do that pretty easily; that might help your application. Rather, that "mighth" help your application.
I called Dartmouth today to ask if they had any special instructions for grad students....they don't. So basically, use whoever you think is best. They will accept more than two, but "will consider your application complete" when they get two...whatever that implies. I'm sending two letters - one undergrad, one thesis advisor, one lab PI.
Thank you for contacting Dartmouth Medical School. We appreciate your interest in our program.
We do not have a preference regarding the people you choose to write your letters of recommendation. Choose professors or employers who know you well and can comment on your ability to be successful in medical school and as a doctor.
You are welcome to send more than two letters of recommendation. However, we only require two letters.