Per the secondary:
"Applicants who have attended a school at which there is no Pre-Professional Advisory Committee must have two letters of recommendation from faculty members at that school. One of these letters should be from a faculty member in the department of the applicant's major, and the other from a science faculty member. If the major subject is science, the second letter should be from a member of a science department other than the person writing the first letter. Letters from Teaching Assistants will not satisfy this requirement, although TAs may co-sign letters with senior faculty. "
It seems as if the other letter must be outside of your major.