Both majors are in the College of Letters and Sciences for UCLA which does not admit by major so select the major you would prefer to study.
UCSD admits into the University first and then into the major. Both Math and Econ are capped majors so it is recommended if applying to a capped major, you select an alternate non-capped major. If you select both for 1st choice and alternate, there is always the possibility you will get into UCSD as Undeclared.