Legal Question in Real Estate Law in California

They don't foreclose on the note, they exercise the power of sale contained in the deed of trust. That means whatever property was conveyed to the trustee in trust gets sold at a trustee's sale. Whatever interest the owners - meaning the owners who signed the deed of trust- had in the property is sold.

Mr. Roach's answer is correct IF the promissory note in question is secured by a deed of trust. However, the question doesn't say whether this is a real-estate financing deal with a deed of trust involved, or whether it is a plain, ordinary promissory note without real-estate collateral via a deed of trust. The general rule is that when there are two makers on a promissory note, the obligation(s) are "joint and several" unless otherwise provided in the note or in some related contract, and therefore the creditor can go after either maker for the whole amount due.