If all the subscribing witnesses are dead, insane, blind, or deaf or hard of hearing, except one (1), that witness may prove the execution of the instrument before the clerk or deputy clerk of the county, the handwriting of the other witness being proved by some other person.
One witness competent
Tenn. Code Ann. § 66-23-104, under Authentication of Instruments by Witnesses.
Tenn. Code Ann. § 66-23-104