This is how I legit multibox d2r, no hacks, no breaking EULA in any way :

Start by having separate bnet accounts with diablo2 purchased on each account. Then In Battle.net app, go to Settings --> Game Settings --> Advanced Features and enable "Allow multiple instances of Battle.net"

Make sure Diablo is installed in a separate location for each account. I have three SSD drives and have one game installed on each drive.

Open your first bnet & Diablo (I always start with the one on local disc C)

I then open a program (downloaded it free from Microsoft) called process explorer 64. This program is great for many reasons, but that’s a conversation for another time. Click the binoculars (search) and type try and find the instance called “check for multiple instances”. It opens after each instance of Diablo is opened. Once found, close that.

Now go to your second installation location (disc D for me) and open it. It should open a new bnet page, log in to your second bnet. It will now update to where your game is opened from (you’ll see your first bnet update as well). Once logged in to bnet, log in to Diablo. Enjoy.

For me, this is when I go back to process explorer 64, close “check for multiple instances” again and open disc E and load bnet #3. I also have my three folders named after the account I want to log in with, in case one crashes I know to close “check for multiple instances” and load that folder. Takes me about 30 seconds per game to do, making it way faster than other methods I’ve tried/seen.

If you have any questions feel free to ask on this comment in case anybody else has the same question!