Usually not... You should have a course number and a grade for each of your "non-science" and "science" letters. In fact, some schools will even ask on the secondary to list the course numbers for each of the letters you turn in. However, this doesn't mean that you couldnt use your PI as a science letter. Best bet would be to call the schools you are applying to and ask them if it is ok. Otherwise find someone else to write a science letter. The basic recommendation for LORs is:
2 Letters science faculty you have had classes with
1 Letter non-science faculty you have had a class with
1 Random letter (PI, mentor, boss at work, comunity service leader etc.)
1 Letter regarding healthcare experience (Volunteer coord. or Physician shadowed)
This package will get you by at most schools. Some schools only ask for 3, but if you send them these it should be ok. Other schools may even ask for a letter from a friend as a "character reference."
So I would try my hardest to find a professor for the other science one. Also when your professors write the letters - make sure you have them mention what class you were in of theirs.
