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When law schools ask for 2 letters of rec, can we give them one from a professor and one from an employer, or is that frowned upon? Currently I only have one strong-ish letter of rec from a professor, and I'd hate to just ask another professor who does not know me that well to throw something together.

Note that I am in no hurry to apply (planning on applying next year, for 2014), but will be graduating this year. I will be working for at least a year before I ask my employer for a letter of rec, and will have worked a total of 2 years before actually leaving for law school.

No, it's not frowned upon.
It varies by school, but generally, the LOR writer should be able to attest to your intellect and growth.
I agree that you should choose your employer, who knows you well, than a professor who will batch something up out of the blue.

Preference for most schools is both be academic. I know for sure that YLS wants academic, even for those in the work force. I know their dean of admissions made this clear in a blog post: http://blogs.law.yale.edu/blogs/admissions/default.aspx

However, outside of YLS, if your out of school for some time or can't snag another academic LOR, then 1 academic and 1 professional is fine. Ultimately, those LORs don't mean squat in comparison to your LSAT, GPA and are below your PS, resume in terms of importance of other application materials.