2 joysticks are obviously needed for cocktails - duh.
In an upright machine, the only games that need 2 joysticks for 2-player simultaneous play are 1942, 1943, and 1943 Kai. All other games alternating, meaning you use the same joystick and buttons on a single joystick control panel. If you set it up with 2 joysticks and 2 sets of buttons, then on a game of Galaga (for example) the first player would use the left set of controls and the 2nd player would use the right set of controls.
Also, there is a setting in Gunsmoke to change it to 1-button play, so you don't have to use 3 buttons if you don't want to. Your character will shoot in whichever direction you are moving. It sucks that way, but it possible.
Also, a trackball is NOT necessary, as you can play the games with the joystick. It's not as good, but can be done.
If you are making these game to sell, then the 3rd button, 2nd joystick (and buttons), and trackball are expenses that aren't completely necessary unless requested by the buyer...