Signing by Another Person
A testator who is physically incapable of writing his or her own signature may have another person sign the will on his or her behalf. When this is done, the other person actually writes the testator's signature on the document.  Any will that is signed in this manner must have two subscribing witnesses and is invalid without them.

Unlike a will that is signed by a mark, these witnesses do not need to be present when the signature is made. However, before anyone can sign as a subscribing witnesses, the testator must specifically identify the document is his or her will and confirm that he or she instructed it to be signed in that manner.

See: Improper Execution, Generally; Signing By Mark; Will Validity, Generally