Double majoring basically means that you are "accepted" to the major the same way that a freshman admit is. You complete the pre-reqs, and if you meet the other requirements for the major, you're in. It's not difficult outside of a few highly impacted major.
Your best bet is to go to each department and look for the requirements (this skill will serve you very well at UCLA, by the way.)
[quote]Transfer students in this category must meet one of the following:
1. Have a minimum 3.5 GPA in Econ 11 & 101 and a 3.3 cumulative UCLA GPA.
2. or Have a minimum 3.3 GPA in Econ 11 & 101 and a minimum 3.5 GPA in Econ 11, 101 and all Economics courses taken by the time Econ 101 is completed (these courses must have Economics 11 as a prerequisite) and a 3.3 cumulative UCLA GPA.
For both transfer and non-transfer students to be eligible for the major, you must have completed all of the pre-major requirements with a minimum 3.0 GPA and a “C” or better in each course. Please note that your grade in the writing course is not calculated into your pre-major GPA, but you must earn a minimum grade of “C.”[quote]
Read the rest of the requirements here: UCLA Department of Economics
Double majoring, as a rule, is not "difficult." It's just a matter of doing the classes required. When you complete your two majors, it will read "with a major in _____" twice on your diploma.