when i originally posted the suggestion of writing 2 letters of intent, i was thinking about the content of the letters very literally. in other words, if you write to both schools "if you accept me, I will withdraw from other schools and attend your school," then technically, you are not obligated to do anything until the school accepts you.
however, if it just so happened that both schools accepted you on the same day (small chance, but it could happen) you'd be totally screwed.
so i'm changing my advice: don't write 2 letters of intent.
HOWEVER, another option is to talk to your premed advisor. Sometimes, advisors have particularly good relationships with admissions officers at certain schools. So, figure out whether your advisor has worked with the admissions office at either GW or BU. Then ask your advisor to call one or ther other on your behalf. This can often be even more effective than a letter of intent or a letter of interest, because the admissions office will know that, if your advisor calls, then they are your one and only. Plus, having a trustworthy person speak on your behalf carries a lot more weight than just another piece of paper begging a school to take you.