Legal Question in Real Estate Law in California

Not without a properly prepared and executed Power of Attorney for those types of document. No exceptions for those sorts of documents.

Yes, he can, although there are problems and limitations. A person who affixes the signature of another to a document with permission and/or upon the principal's instructions is called an "amanuensis" (a "legal Latin" term).

California has a rather well-developed "amanuensis rule" that allows third parties to affix the signature of a principal to a document such as those you describe in certain circumstances. In general, the person signing as an amanuensis must perform the signing as a purely mechanical process, following instructions from the person whose name is signed, and may not exercise discretion in the matter, as an agent ordinarily does. Further, if the person acting as the amanuensis personally benefits from the transaction, a rebuttable presumption arises that the signature is fraudulently applied, and if questioned in court the amanuensis has the burden of proving that he or she did in fact sign upon the isntructions or authority of the principal. The amanuensis rule was discussed in a California Supreme Court case, Estate of Stephens (2002) 28 Cal.4th 665, which you can look up on line.

I think it is preferable to use a power of attorney, as Mr. McCormick says, and even then not everyone you do business with is going to find this acceptable. They'll say "No, get her in here to sign in person!" or demand that the power of attorney be granted on their own form. However, in the right circumstances, your husband can sign on your behalf when following your explicit instructions, and vice-versa. Finally, note that married couples are in a fiduciary relationship and legally owe each other the utmost in fair dealing, honesty and clarity of communication regarding financial, business and contract matters.

Only 'on your behalf' with a properly done Power of Attorney.