Signing By Another Person
A declarant who is physically incapable of writing his or her own signature may have another person sign the mental health living will on his or her behalf. When this is done, the other person actually writes the declarant's signature on the document.

All mental health living wills must have two subscribing witnesses and is invalid without them. However, neither of these witnesses may be the same person who signed on behalf of the declarant.

See: Signing By Mark; Witnesses, Generally