I'll be damned, you learn something new every day.
"However, at least one federal magistrate has decided that if
the grip is added later, the gun is not 'originally designed' to be
fired by holding in more than one grip, and thus putting a second
grip on a pistol does not make it an AOW. ATF does not regard the
decision as binding. The case is U.S. v. Davis, Crim No. 8:93-106
(D.S.C. 1993) (Report of Magistrate, June 21, 1993). The
prosecution was dismissed at the request of the Government before
any review of that determination by the trial judge."
So whether legal or illegal to do so (which is up in the air) a second grip is installed at the owner's peril.