The letter of reference and its strength doesn't play into the decision very much. You will be a stronger candidate if you have two decent academic letters, fulfilling their request, rather than an excellent professional reference. They're looking to see if you are a good student, and right now you have the capacity and proximity to develop relationships with professors that is much stronger as opposed to someone who has been outside of school for a few years, which is why they are asking for two from you. Additionally, the letter from a professor might be better than you think as they have given and received tons of letters over the years so they are well versed and know what admissions are seeking. Professors want to see you succeed, so I'm sure it will be better than you anticipate. BTW this info came from an LSAT prep course I took last year.
For myself, I contacted an old professor of mine that I only had one class with, and I wasn't sure if he would even remember me. I gave him a few pointers to remind him who I was and although I never saw the letters, I got in to a few schools no problem.
Thanks. This is reassuring. I'm really panicking about the letters because I did not develop close relationships with any of my professors and was really banking on only having to ask one....
If anyone else is wondering, I explained this to Dal Admissions and this was the response I received:
"The minimum number of references is, indeed, two. Of these, if you have been in school within the previous three years, they should both be academic in nature. However, if you have an additional, non-academic, reference, you may certainly submit it as well.
In addition, if you are in a position where you can only obtain one academic reference, you would have to submit an addendum letting the admissions committee know why you will only be submitting one academic reference."