For entering the job market after such a program, it is indicated that the rewards from the second major are not worth the time and effort (and perhaps other expenses) of doing it. A look at jobs websites shows there is (some) demand for bachelor’s level biochemists (to run assays) and (some) demand for bachelor’s level mathematicians (for instance, for marketing data analysis, actuary work with a ASA or FSA credential, and education with or without a teaching credential), but no demand for bachelor’s level biochemist-mathematicians.
There are expected to be more job opportunities in the mathematical area than in biochemistry (http://www.bls.gov/emp/ep_table_102.htm). Also, many more jobs are expected in certain areas of computer science (for instance, software development and computer systems analysis) than in the more strictly mathematical occupations; job-wise computer science makes more sense.