Basically, applicants to USC are being evaluated on 2-3 fronts… by the university overall and by the individual School or Schools within USC that represent the first and second choice majors selected. If the School or Schools want an applicant, but USC overall is reluctant, that School may have to fight for an applicant. That scenario is usually more common in the creative and performance Schools. The # that they can do so for in an individual cycle is likely very limited… so it is usually the other way around.
If USC and both programs want the applicant, he/she will be admitted and to their first choice major. If USC and only the second choice want the applicant, he/she will be admitted and to their second choice major. How often it happens likely just comes down to the combination that the applicant listed. I presume it happens rather often.
Based on your choices, it may simply depend on the #s admitted to each program and how those programs rank you versus other applicants they want. That is also how Spring admits emerge. If USC wants to admit you and so does a program, but there aren’t quite enough spots available… but spots are available for the Spring… an offer of admission starting in the Spring may be forthcoming. It just comes down to how they rank you specifically for that program and where that ranking fits into the # of spots available for the Fall vs the Spring.
And yes… USC does admit a good # of students each cycle as undecided. My younger daughter (a current senior) was one of them. And during orientation, she was put into and met a group similarly admitted. In her case, both of her major choices were highly competitive SCA majors. Both passed on her, but USC saw fit to admit her Undeclared. She enrolled anyhow and applied for an internal transfer into SCA over the summer in July and was admitted to her SCA major by October. Generally speaking, it is far less competitive trying to internally transfer – even into the most competitive programs - versus applying originally as a high school senior. So I would not let getting admitted undeclared scare you off if USC remains your top choice. My daughter even passed on Princeton, who eventually admitted her off of the waitlist, to attend USC - even as undeclared initially. So she still started in the fall, even taking a couple SCA courses, and was firmly entrenched within her major officially by the Spring.
In terms of comparing applicants who got in versus those rejected or even comparing those admitted for the fall versus only for the spring, it is virtually impossible for others to predict what may or may not occur in any specific case. Applicants often boil it down to simple stats when comparing themselves to other applicants, but USC is clearly evaluating much more. With an admit rate this cycle that will be in the range of 11-13%, and with 4K+ applicants with 4.0 unweighted GPAs and test scores in the 99th percentile among those not gaining admission, any individual applicant’s success will instead likely depend on too many potential factors for anyone to correctly evaluate… making predicting admission nearly impossible. But this also shows that they admit many without perfect grades or test scores. They are after all seeking to craft a well-rounded and diverse freshman class comprised of those who actually have a strong affinity for or connection to USC. They do aim to admit those who they project will thrive well at USC and bring something unique to the campus environment.
An admission decision will likely come down to a composite and holistic analysis of your stats coupled with writing ability / essays, ECs, potential leadership roles, potential other unique qualifiers (URM, First Gen, geography, demographics, etc.), your Why USC? explanation/reasoning, etc. And that “Why USC?” explanation is likely the most important single component of your application. You do need to provide a well-thought out and well-researched answer as to why attending USC is truly significant and important to you specifically. And moreover, USC does want to gain a sense as to what you will be uniquely contributing to the greater USC community if admitted.
Good Luck…