Legal Question in Real Estate Law in California

As between the landlord on the one hand and A&B on the other, A and B EACH are jointly and severally responsible for the FULL rent to the landlord. If BOTH move out, they are only responsible for the rent until the landlord can re-rent the property. As between A and B, they are each responsbile for 1/2, and if the landlord forces either one to pay the full rent, then the one who pays is entitled to 1/2 reimbursement from the other. If the one who didn't pay their 1/2 refuses to reimburse the other, the other can collect in small claims court.

I assume Party A and Party B are both tenants on the lease, and there is a "Party C" who or which is the landlord. In that case, unless the lease terms are quite unusual, both A and B are liable to C for all the rent. C cannot double-collect, of course, but C can choose to collect from A or B, and will usually go after whomever is still in the rental unit

If B has to cough up an unfair share of the rent because he/she was sued, or felt obliged to pay to avoid eviction, or for any other reason, B is entitled to recover the excess from A.

Exceptions are possible under unusual circumstances or perhaps a side effect of some oddball local rent-control law, but these are the general legal principles govering.