Here are the Model 2 cab measurements from the driving cab info thread:
(This is from a virtual on cabinet. The driving cab seat risers are a couple inches shorter)
So the footprint is 28.75" x 59.5"(This is without end caps. We'll just call it 30" x 5').
The front part overhangs the base by about 4.5" on each side.
So if two cabs were pushed against each other in a twin setup, you're only looking at 9 or 10 inches between the bases for player 2 to enter and exit the seat. The Daytona Twin manual says it's 62.5" wide including the endcaps, so there might be an inch or so more.
If you have the space though, I prefer to have twice that much between the bases of the cabs.
Games like Virtual On had the players enter seats from opposite sides, but that's not as easy when there is a shifter in the way.
(and no, I'm not playing with the shifter on the "wrong" side)
Also, if the cab is bordered on one side by a wall, I don't like having the cab right up against it.
For me, it's much more comfortable to leave 3 or 4 inches between the side of the cab and the wall.
So in summary (and IMO):
Standalone sit down cab: 30"x60" (add 4 or 5" to the width if one side is up against a wall)
Twin Cab setup with original spacing: 63"x60"
Two single cabs with my preferred spacing 73"x60"