Do you mean multi boxing? If so it is not supported (they won’t help you set it up), but it is not prohibited as long as you don’t bot or use automation.
The old set up was to have multiple computers connected to one mouse and keyboard so all the copies of the game did the same thing at the same time. Now there is software to do that on a single PC so that your keystrokes and mouse commands in Windows are sent to each game client. You still have to control every movement and pick up all your loot, etc.
It is time consuming to be honest and can slow you down.
However, if all you want are achievements you can probably park the second character out of the way somewhere and complete the achievement. That won’t get you in trouble.
HOWEVER - there is an update you need to be aware of regarding Diablo 3 and the Bnet Launcher and password logins Password Update - April 15, 2020
If you want to run two accounts you will need to make sure your password is not too long and in the Launcher, enable multiple copies of the launcher to run.
There are some here who multi box who can give better directions, but that should cover the basics.
You were but I was typing a lot! Food is later though. Not quite hungry yet. 
