It would seem as if you didn't know three people who would recommend you. Find a third, but make sure they'll write as though they know you well. When I applied to grad school I was in the same boat (1 internship supervisor and 1 prof. I knew well enough to ask comfortably). I had to go back to a professor who only vaguely remembered me (he probably only remembered my face, not my name). Most profs will write you a solid reco, just give them your resume and some details about your life/plans for the future. Be sure to write thank you cards to them (I would also include a gift card or something for their time). Many profs have pre-made recos too.