Hey OhioDoc, thanks! I think my description of my situation may have been confusing.
First school:
Research experience for 1 semester + summer, letter from post-doc
Second school:
Research experience (minor, but I put it on AMCAS), no letters because my post-doc left for France, and no letter from the PI (I was really just assisting the post-doc with his project)
Research experience where I'm completing my senior thesis, letter from PI
2 summers at the NIH i n one lab, letter from PI
I just want to make sure this doesn't make adcomms think that I had a bad research experience at my second school-- it was just that that post-doc left.