California
Probate Code – PROB § 4122
If the power of attorney is signed by witnesses, as provided in Section 4121 , the following requirements shall be satisfied:
(a) The witnesses shall be adults.
(b) The attorney-in-fact may not act as a witness.
(c) Each witness signing the power of attorney shall witness either the signing of the instrument by the principal or the principal’s acknowledgment of the signature or the power of attorney.