Some people like more space than others. I've seen
36" wide panels for two players with the joysticks near the edges, as well as wider.
Me, I grew up with 24" 2 player CPs, and enjoy it (as long as the guy next to me doesn't stand with his feet farther apart than his shoulders; damn "cowboys"). IMO, those 36" panels put the players too far off center (unless it's a showcase cab), whatever rocks people's boats.
A lot of it is personal tastes, but some things to think about: type of cab (showcase lets you go wider), size of cab (smaller ==> smaller CP), size of monitor (small ==> smaller CP), distance to monitor (closer ==> smaller CP), number of buttons (more ==> bigger CP), one player two stick (tank) games or not (yes ==> sticks ~ 14"-19" apart), size of yourself & your friends (bigger ==> wider CPs), and size of your "personal space".