A form of revocation must be executed—
(a) by—
(i) the principal signing the form; or
(ii) a person, who is eligible to do so under section 106, signing the form in the presence of and at the direction of the principal; and
(b) by one person who—
(i) is present and who witnesses the principal or person signing the form; and
(ii) who signs and dates the form in the presence of the principal.