Well the second joystick would really only cover the face buttons (A, B, X, Y or Triangle, X, Circle, Square). It only has four switches.

Yes, you can map the diagonals but that's really only hitting two "face" buttons at once. And likewise, you can't add shoulder buttons" to the first joystick. There's no where to put them.
And practically speaking, if you needed to work the shoulder buttons in line with the joysticks, it'd be almost impossible to play. What it you needed to move while hitting the shoulder button.
These dual-joystick setups are for games where one stick is move and one fires. The second joystick would not work well for, as an example, Street Fighter II or anything requiring more than four buttons (in reality two since you cannot physically move the joystick in two opposite directions at the same time whereas you can hold down two buttons quite easily).
Unless this setup is strictly for Robotron, Smash TV, etc. it's not very practical.