Usually you'd install both discs to two folders then move contents folder of disc 1 to contents folder of your HDD so when you launch disc 2 it'll actually load properly. The method you're thinking is installing it as if its a retail console both discs which would work but you wouldn't be able to mod files or really do anything to the game via Xex menu. Could only use a plugin menu or tool.

I would suggest looking on YouTube, for GTA V just lookup how to install GTA V on rgh and it'll show you how it's done.

If you downloading the ISO:
Extract with xbox image browser then:
all you do is copy the install disk to the Content folder on your HDD
or you could even copy the install disk to HDD and run the default.xex of it and it will install.
and then you can copy the play disk to HDD and it will work fine.

If you got the real disks:
For 2 disk games its pretty much the same as a normal console.
1. Install the Install disk - it copys all the files to the right place (content) anyway.
2. Copy the play disk to hdd with xex menu (theres tutorials on yt for this).

if this doesnt work then you may have to download the play disk ISO
Extract with xbox image browser then:
copy that to hdd instead.