It is legal, because it isn't prohibited. A forward pass after the ball has been in possession of an A player completely beyond the NZ is prohibited by rule. A second forward pass is prohibited by rule.
A scrimmage kick (one or multiple) made from in or behind the NZ after a previous scrimmage kick, after a fumble recovery, after a pass (forward or backward) has been completed, or after the ball has been in possession of an A player beyond the NZ is NOT prohibited, as long as a previous scrimmage kick has not crossed the NZ, and there has been no change of team possession.
The only one of those things I've seen is a Tulsa punter that tried to run for a first down, saw he wasn't going to make it, backed up behind the NZ and punted. Legal.
Would be cool to see video of the double kick play.
Robert