Hello, this is the case. I want to earn some coop achievements. I am mostly a solo player.
What I am asking is: Is it ok to run 2 different battle net accounts at the same time, having a party and earn the achievements this way?
The reason I am asking is because I don’t want to get banned for such a silly reason
It’s called multiboxing and yes it’s fine.
HA I’m faster than a cheetah at suppertime 
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. 
Oh you mean 3rd Party software that by definition breaks the TOS.
No, it does not. Anything that is NOT D3 is third party. Discord, your music player, your OS, your web browser, etc.
As long as it does not touch the game, game memory, or game communications, it is fine.
Please don’t mislead people.
I do not know any game company that says you cannot buy two copies of the same game. Some may even encourage it.
I can only assume you have two separate login names and two different passwords. While an I.P detector is moderately efficient, I’d assume you were also running them on different machines? Theoretically it should not matter as the owner of both copies of the game, but probably a good idea if not in case something changes.
As said in other above posts, if you are not cheating by using a robot to do the work for you, then I cannot see any issues. You simply have two copies of the game and are playing them simultaneously on different computers.
Funny thing, that is exactly what I am doing. Bought another copy on a new account, since the game is on sale and I wanted to complete the co-operative achievements. I have another computer to run it on nearby. Not really a game machine, but with all graphics options set to low, it runs ok, and I just need the other character to be in the game, so it doesn’t have to move around.
I run two instances on the same computer often. I normally have two monitors to make it easier, but if you run the game in windowed mode, (or windowed mode full screen) you can alt-tab between the two games very easily.
I have run up to 4 games at the same time, but there are some things to keep in mind If you are going to run more than one copy on the same computer:
1: Memory. Make sure you have a MINIMUM of 8GB of memory. D3 has two game types, 32 bit and 64 bit. If you have 8GB or less memory, you will want to run one or both instances in 32bit. D3 can be a memory hog, especially if you run back to back rifts. One way around it is to leave and restart the game every so often. (the session, not clear out of the game)
Running even 2 games with the 32bit exe may be troublesome on a system with 4GB or even 6GB of memory.
2: There are multiple ways to log into D3. I myself use my main account with Bnet. While I can run multiple instances, I normally don’t. I just run the second instance from the EXE directly with a custom shortcut to disable the launcher command.
3: No automation. Any kind of software used to automate the game (as in control one or both sessions for you) is prohibited. That said, you CAN use multi-boxing software (usually obtained 3rd party and for a price) that will allow you to operate multiple sessions of D3 simultaneously. You click to move the character and both or more games move the character the same. You activate a spell slot and all games activate the same. This is NOT automation. Lot of people try to say it is. However take it from someone that has multi-boxed for over a year. Its a big fning pain in the a and not really worth it in the long run. Hardest thing is keeping the games in sync with each other. Switching between games is far easier and free.
4: You do not need individual installations. Your one installation is sufficient to run multiple copies. (I have personally ran as many as 8 separate games at the same time)
5: Having more games open will put more demand on your system overall. Be sure to shut down as many things as you can that you don’t need running in the background. This kinda goes along with 1.
This is a video showing how I have mine setup to run extra instances. You can pause the video to see how the shortcuts are setup if you wish.
Good luck.
Clever use of your machine.
Yes, one person can play 2 separate copies of the same game.
Don’t be confused with having two people play the same account. That’s not permitted unless you are a parent sharing an account with a child.
Some people are going off topic. I don’t think the OP is asking about how to do it…
Those coop achievements are easy. I made most of them going in public rooms - Rift or Bounties. While other run their rift, i go make the bounties or whatever.
The problem comes from some boss killing that can happen ONLY in Campaign. So when i found the boss in campaign, i asked a friend to join me and teleport to me, then i kill the boss and get the achievement for coop kill on that boss.
It’s even easier to dual/multi box on console. Just need extra controllers and PSN accounts. The funny thing is you don’t even need to control all the characters as they will auto follow whoever is being controlled.
Anyone know the code for terminal to launch second instance ?
Its in the video I linked above. Just change the path to your game to match what is there. Mine runs from my C: drive, since its an NVMe SSD. (Faster)
Normal game path is C:\Program Files x86\Diablo III\ for reference.
Make a shortcut to the Diablo.exe 64 bit (or 32) like below:
Target: "C:\Diablo III\x64\Diablo III64.exe" -launch
Target: "C:\Diablo III\Diablo III.exe" -launch
Make sure the “Start in” window has the same path:
Start in: "C:\Diablo III\x64"
Start in: "C:\Diablo III"
But on your system replace the location (mine is C:\Diablo III) with whatever your path to Diablo III is.
Wouldn’t happen to know the one for Mac Terminal ? should of been more specific , thank you thou .
Sorry, no. Tias might if he happens to visit the thread.
Do you need to install it 2 times on computer aswell?
All i get one i launch the second game from a paid version of it is “Playing now” is grayed out and not able to play second account.
No.
Make sure you setup the shortcut as I described above:
Here is the video where I show the shortcut directly:
Just need to have it setup correctly and it should work fine. Make sure you use the path to the D3 folder that your system has. It may differ from mine. The default directory is:
*C:\Program Files (x86)\Diablo III\x64*