You have to have 1 complete installation on Diablo II for each window you want to run. For example if you want to run 3 Diablo II's you must have 3 Diablo II's installed in different directories on your hard drive. (It does not matter what you named the directory or the location of them, just make sure they are in different folders) They cdkeys you use must be the default ones you installed in your Diablo II installation directory. (You will not be able to use any cdkey.mpq files that you made with Onlyers's cdkey changer)
Assuming you use Windows XP, go to your control panel. Click on User Accounts. Make separate user accounts for each Diablo II window you want to have loaded. For example my User Accounts are JMH, JMH2, JMH3 on my computer. This allows me to have 3 D2's active at once. If I wanted to have 4 up at once all I would have to do is create another user account and install a fourth Diablo II on my hard drive. (You will not be able to load more than 4 Diablo II windows because blizzard has set this as the maximum number you can have per IP address that are connected to BNET at the same time) Once you created all the accounts, click on change the way users log on or off. Make sure you have the Use the Welcome Screen and Use Fast User Switching boxes both checked. Then close the user accounts screen and your control panel screen.
Start one Diablo II game then alt tab out of it, click on start menu, then click log off, then select switch user, then click on whatever user you want to load. (The first time you switch to a different user account it will take a minute to load each one, after it has loaded once already it is a very quick process) Then start another Diablo II game that is loacated in a different directory with a different cdkey as the default. Repeat this process for however many games you want active at once.
This method is 100% legit. It does not alter any D2 files or use any other programs that Blizzard doesnt like us using. There is NO way that blizz can do anything about it. You did not alter their files or use any 3rd party programs. Now we can have as many D2 windows up at once and not have to worry if blizz will do anything about it No more worries about droping due to timeout (except for the usual ones that blizz gives you every once and a while becasue of server lag).
With hex editors and d2loader you run the possibilty of Blizz banning you because it viloates the EULA we all agreed to when we created our BNET accounts. (Hex editors alter Diablo II files and thus is in violation of EULA, and d2loader disables the cdkey check which is a copywright violation)
As i said earlier, I did not discover this method, this guide is based off a method that was posted by Inokis on mousepad's forums.