<p>What do you mean by "how "</p>
<p>If your question is of “technicalities” then you have to go and file a petition at the counseling office.</p>
<p>So for UCLA- as an undergraduate you go to counseling in Murphy Hall after you completed all the prequisites for both majors and 2 upper division courses in each major and meet with a counselor. They will ask you to make an educational plan of sorts and fill out a petition - then they review it and if it is approved your DPR (degree progress report) on the online system gets updated stating both majors and what you have left to complete for both majors.</p>