Power of Attorney, Signing By Another Person
A principal who is physically incapable of writing his or her own signature may have another person sign the power of attorney on his or her behalf. When this is done, the other person actually writes the principal's signature on the document.

Any power of attorney that is signed in this manner 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 principal.

See: Signing By Mark; Witnesses, Generally