Doesn't matter. If you've identified as AA your entire life, mark both your ethnic components. If you've never really identified as AA and you're just doing it now for law school, that might get a little sketchy. Regardless, you're in the right by marking yourself down as a URM.
That's a lot sketchy to me, so the most honorable thing to do would be to mark AA if you've considered yourself an AA in the past, if not, then you shouldn't. But, honor tends to get pushed aside when access and $$ are involved, so I can understand how checking the box would be hard to resist.