First of all, I see an "in the area" email as a variation on a letter of intent. Remember, you are still trying to sell yourself to the school. Thus, I think it's a good idea to put in your "in the area letter" specifically why you want to interview there. Here is a format I'd suggest:
[Salutation],
[Paragraph about "being in the area," and how much you'd like to interview at GW also and (briefly) why it is difficult to make a trip to DC twice.]
[Paragraph with more details about "why GW."]
[Conclusion paragraph; recap how much you want to go to school here and why]
[Closing]