Not normally, at least in Pennsylvania. This question comes up surprisingly often and usually in the case where there is a “homemade” will (that is, one prepared without a lawyer).
In Pennsylvania the generally applicable signing requirement is that “(e)very will shall be in writing and shall be signed by the testator (the person making the will) at the end thereof”. There are two exceptions to this: (1) if the testator signs by mark and (2) if someone signs the will on behalf of the testator, because he or she is too weak to make even a mark. In those two cases, and those cases only, two witnesses do need to sign the will.
In the case of an un-witnessed will, two witnesses (called “non-subscribing witnesses” because they have not written or “subscribed” their name on the document) must swear that the signature on the will is that of the testator. To be clear, the two non-subscribing witnesses do not have to swear that they saw the will being signed; they need only say that they are familiar with the testator’s handwriting and that the signature on the will is his handwriting. Usually, these witnesses must be present at the Register of Wills office to take oath when the will is to be probated.
Why then do we require two witnesses to sign twice (and in the presence of a notary) when clients come in to have wills signed? First and most obviously, if a will is un-witnessed, there is an increased chance that a third party could claim that the testator’s signature is a forgery. Second, if a question later arises about the testator’s mental capacity at the time of signing, the names of witnesses who can rebut this theory are located right on the will. Third, if the witnesses sign the will and a notarized affidavit, it eliminates the possibility that someone will need to come in to the Register of Wills to later “prove” the signature. Fourth, Pennsylvania is unusual. There are many states that require a will to be witnessed. A properly witnessed will increases the probability that the document can still be probated if the testator later moves to another state.
We recommend that all wills be signed by the testator and witnessed by two witnesses with all the attendant formalities. However, if a loved one dies with a signed but un-witnessed will, there is no legal basis for ignoring it; it should still be probated.
– Rod Fluck