i'm sure adcoms prefer math majors to econ majors
i don't think that a double major in math and econ is worth the effort, because i doubt that the gain is substantial. there are probably more efficient uses of your time (ie math, stats, graduate econ courses, personal time, etc)
edit: also, i'm not sure that an undergraduate math for economists class would be useful for a math major, and probably does not help much in the admissions process.
also, i'm sure there is someone at your university that has a better understanding of the admissions process than we do. my best advice is to ask them for help.