Attach a "pistol" to a "pistol"

So an AR-15 pistol is exactly that, a pistol.

The Taurus Judge in .45 LC, although capable of firing .410, is a pistol.

Can anyone find a legal reason a person could not use a Taurus Judge as a .410 masterkey on an AR pistol?

I thought it might constitute a combination gun and thus an AOW, but near as I can tell a combination gun is legally one firearm, not two.

I am aware this is stupid. Embrace the meme. Question not why, but if one can.

Problem 1: The judge's grip is a vertical foregrip. Solution: birds head style grip on the judge. It remains a pistol, and it's grip is an AFG on the AR pistol.