Your main home is where you live most of the time. It is certainly possible for a married couple to have 2 main homes, one for each spouse, but that means the spouse's mostly live apart, and only occasionally visit each other. It's uncommon, and you should expect that if you claim tax benefits for having two main homes, the IRS or FTB may want you to answer some detailed questions about your living arrangements.
For example, your job is in city A, and your spouse works in city B, 100 miles apart. You rent an apartment in city A and stay there Monday to Friday. You drive to city B Friday night to stay with your spouse and you drive back to city A on Monday. While this situation must be judged on all facts and circumstances taken together, let's suppose that your friends, doctor, dentist, church, favorite car mechanic, and other important relationships are all in city B. You maintain a closet full of clothes in city B, and you are not planning a marital separation. If you lost your job, you would be more likely to move back to city B to look for a new job, rather than stay in City A. While city A is your "tax home" (which is a term that is important for certain deductions and credits) I would say that city B is still your "main home."
Anyway, that's the kind of analysis that would be applied.
