Hmm...I double majored (philosophy/psychology) from umich and have had success beyond what I'd anticipated re: admissions, so here are my thoughts (and the thoughts of all the applying to law school books I've perused): If one of the majors was much more central to your undergrad than the other (i.e. you wrote a thesis in it, were involved in activities within the field), then it's definitely acceptable to have your PS/LORs reflect that.
However, I would think you are short-changing yourself by not going for the more diverse approach. Adcomms have stereotypes about the strengths/weaknesses of all the different majors out there...having a background in two different topics, therefore, adds to your overall strength and helps cancel out your perceived major-related weaknesses. In my case, I wanted my phil recs to show that I have solid writing/analytic/argumentative skills and my psych rec to establish that I was personable, able to articulate myself in presentations, and work in groups (flaws typically ascribed to philosophy students). I definitely focused more on my phil degree, so had 2 recs from it, 1 from a psych prof, and a final one (for schools that allow 4) from my academic advisor.
BUT, all of this is moot if you don't think you have a prof in your 2nd major that knows you well enough to write a solid recommendation. If you have two profs that you think would be on par, go for diversity. Otherwise, stick to the ones that will be of highest quality (although, perhaps, suggest that the two profs focus on different aspects of your awesome-student-ness).