Quote (neosoph @ 17 Nov 2011 10:59)
1) Install Diablo 2 as you would normally using your 1st CD-Key.
2) Copy the entire "C:\Program Files\Diablo II" directory onto your desktop.
3) Change the name of the folder you just copied: "C:\Program Files\Diablo II" to "C:\Program Files\anythingelse"
4) Re-Install Diablo 2 using your 2nd CD-Key (you may be forced to uninstall first depending on your operating system).
5) Cut what you copied to your desktop and paste it in the "C:\Program Files\" directory.
6) Voila! You're done. Simply play the game from either directory to change Cd-Keys. You will not be able to play both at once.
You really only need to change the name of Diablo II to anything else, no need to copy and paste to desktop.
So for the already installed Diablo II, rename to "Diablo II-1", reinstall game and then rename that to "Diablo II-2".
The installer won't know that you renamed your Diablo II folder to Diablo II-1 so no need to copy and paste, since it goes by the Installation Dir in the registry. It can't find that path so it thinks you removed Diablo II.