Does applying to UC San Diego improve my admissions chances compared with UCLA for the UC application?

The biggest practical tradeoff is selectivity: applying to UC San Diego does usually give you a better admission chance than applying to UCLA, but both campuses are still highly competitive and the UC application does not let one campus “boost” another. Each UC campus reads and decides separately, even though you submit one application. So adding UC San Diego can improve your overall odds of getting into at least one highly selective UC, but it does not make UCLA more likely.

For admissions chances alone, UCLA is one of the most selective UC campuses, while UC San Diego is selective but typically a bit less difficult to enter overall. That means a student who is borderline for UCLA may still be reasonably competitive at UC San Diego. The difference is real enough that many students apply to both for that reason.

The main thing to understand is that there is no shared admissions decision across campuses. Your PIQs, coursework, grades, and activities are sent to both, but UCLA and UC San Diego evaluate applicants independently and can reach different results.

Major choice matters too.

If your question is simply whether UC San Diego improves your odds compared with applying only to UCLA, the answer is yes. If your question is whether UC San Diego is easier to get into than UCLA, the answer is also yes in broad admissions terms, though not by enough to treat it like a safety.