Rockstar says for 360 version do not install both discs.

[attaboy]

I can't think of a single reason for this. I don't like it, though. I'm sure it wouldn't happen often but I remember how pissed I was when I played Metal Gear Solid 4 and I was ready for the game to continue. Instead, I had to wait for the game to install. I think I'll get flashbacks with GTAV.