Some times that's not even good enough. I called to check on my application at Tufts a few months after they said my application was complete and they told me that my application wasn't being considered yet because I used 2 chem professors and didn't include a bio professor. I had submitted them in a committee letter and their website said:
"Required Letters of Evaluation:
One letter from a Biology professor and;
One letter from a second science professor - Biochemistry, Chemistry, Math or Physics
Or
A pre-professional composite committee letter"
Since I satisfied the "or" I figured I was good, but I wasn't and they wouldn't have told me if I hadn't called. So be very careful.