Question for Harvard acceptees/applicants regarding RECOMMENDATIONS, Please help

I have a question regarding Harvard applications. I had 2 research positions. 1 position for 2 months during a summer, and the other for 1 year and I'm continuing next year. I Have a rec from my 2 year research position, but not from the 2 MONTH position. The supervisor likes me, but I really don't know him at all, and I don't want to ask him for a rec. Does Harvard REALLY want a rec from every research prof? Has anyone done this before?
Thanks