A brief article on Eurogamer has said that Rockstar is warning gamers not to install both disks on the Xbox 360. GTA V comes on two disks, an install and play disk, it’s recommended you do NOT install the play disk.
At the moment, there are just tweets to people who have asked about the installing of the game…
@justinkeppler @Insanely_Bored @EliDrinksZona An 8GB install is required. For optimal performance, we recommend not installing the play disc
— Rockstar Support (@RockstarSupport) September 16, 2013
So the deal is, just install the “Install” disk and play the “Play” disk in the drive.
Source: Eurogamer
Rockstar Support say they will include the issue on the support site.