To be honest, I think Agassi believes this because he's a closet 1H BH fan. Poor guy's just in denial!
Agassi's 2H BH is more of a 1H BH "with support."
1) You basically start with a very classical 1H BH leading with the elbow and tracing a U.
2) With your non-dominant hand, you don't so much grip the racquet as you let it rest in the palm of your non-dominant hand. In fact, it helps to try this with an "open" hand initially.
3) You swing as you would a normal 1H with the dominant hand (legs, straight arm holding racquet, chin over shoulder, etc.) The non-dominant hand follows along, providing additional support.
4) What you find is that the additional support "compresses" and "flattens" the swing. The contact point moves closer to you than what's typical with a 1H BH. The U-shape looks more like a straight takeback. And your finish is over the shoulder.
5) You can experiment with different grips, and vary the grip of the non-dominant hand to adjust topspin between DTL and deep crosscourt shots.
6) Some people notice that it looks like Agassi has wristiness with his 2H BH. But it's a bit of an illusion and it's a natural product of this stroke. You'll automatically see it if you take the racquet back far enough, so that the chin is over the shoulder and the butt cap is facing the net.