A few members here have twin driving cab setups, but they've all been conversions, all side by side.
The Logitech wheels will work perfectly with PC/PS2/PS3, so you can have all those in one cab if you like.
There isn't a PC driver for the xbox360 wheels. They'll work on a PC, but there will be no force feedback.
I'm in the process of building a pair of matching driving cabs, but it might as well be a conversion as I'm using Sega Model 2/Naomi bases and dashboards.
Mine will be PC only. I've picked up at least a dozen good pc racing games for next to nothing in the past few months. Most were $5 or less. Even recent games like Dirt 3 have been on sale for less than $7 recently. Most activate on Steam, some on Origin.
For me, whether to put them opposite or side by side would depend on the room they're going into.
I see what you're saying about looking at the other persons screen, especially for games like Virtual On (which had tinted glass between the players).
It's more of a group/social thing if you're side by side though.
And then there's SailorSat:

He also has his own MAME build with networking.
http://forum.arcadecontrols.com/index.php/topic,130136.0.html