How do the dual Neo Geo "D" button pins work?

[martianviking]

I understand from the Neo Geo controller pinout  page that there are two pins on the standard Neo Geo controller pinout that both represent the fourth button.  This button is labeled "D" and is colored blue on the Neo Geo CD control pad (it is also colored blue on standard MVS arcade control panels).

If I understand correctly, then both of these lines (pin 4 and pin 9) are "closed" when the fourth (or "D") button is pressed.  So, if a controller socket had these two pins wired to perform two different functions (i.e. pin 4 to JAMMA button 4 and pin 9 to Capcom kick harness button 6), then both of those functions would be activated when the "D" button was pressed on a standard Neo Geo controller that was plugged into that socket.  Also, if either pin 4 or pin 9 was removed on a Neo Geo AES (home) system, button "D" would still work, so long as the other pin was still functioning.

This leaves me with two questions:

(1)  Is this as simple as it sounds, or is there anything else that should be noted when working with this pinout?
(2)  Does anyone have any idea why SNK decided to have two pins for the "D" button?

[NFG]

1. Yes, it's that easy.
2. SNK did a lot of strange things.  A mystery for the ages.

[Fattyfromthefuture]

I noticed when you leave both "D" lines unconnected to each other (on the home system), the Debug bios behaves differently (up/down dont work properly instead you have to use A/B to navigate though the menu) If I remember correctly.....I noticed this years ago.

[NFG]

Sounds fishy to me, since until the D button is actually pressed the system doesn't know if they're tied together, or if there's even a controller plugged in at all.

[martianviking]

...(2) SNK did a lot of strange things. A mystery for the ages.

Indeed.  Point taken.

I noticed when you leave both "D" lines unconnected to each other (on the home system), the Debug bios behaves differently (up/down dont work properly instead you have to use A/B to navigate though the menu)

I don't know anything about that, but I have to admit that I'm not as worried about it since I'm mostly interested in this pinout from the controller side (even though I phrased part of my example from the point of view of an AES).

So, now I'm considering usinging a new/modified version of the "Super Neo Geo Pinout" for a SuperGun project that would allow for only 7 total buttons (it would omit the pin 8 with +5v on it).  It would really only need 4 or 6 buttons, depending on whether it is hooked up to SNK or Capcom board types, but of course you need two "number 4" buttons (since JAMMA/SNK "fire 4" and Capcom "kick 4" use separate leads).  Because this is a SuperGun-centered controller pinout, it's mostly centered around 4 and 6 button layouts (and isn't as concerned with maintaining the perfect 8 button compatibility with existing console controllers).

This pinout would use pin 9, as in my example, as a "kick button 6" line.  I'm figuring pin 2 for "kick 4" and pin 10 for "kick 5."  I know this isn't exactly in order, but if someone plugged in a standard Neo controller to a system connected to a Capcom fighting game, I figure that if they could only pick one kick button, they'd probably prefer that it be heavy (roundhouse) kick.

Thanks for the help, and I'd like to mention that I'm very open to further input.