My cab is exactly 28" across and its perfect for two players.
(Far enough for elbow room yet close enough to push your competitor if he's beating you).
I agree with Pointdablame...spend a
lot of time planning out your control panel. This will make your cab a joy to play on or a frustrating game of arm twister.
I noticed on your cab plans that you have buttons on each side of the players joystick. This may make gameplay difficult (especially if you're playing a fighting game).
Apart from that....your cab looks....well.....it looks like mine

Just my $0.02 worth.
Good Luck.