Having multi-boxed for nearly 2 years with software, and a number of years without special software, I can say officially that its more of a PITA than its worth.
Even with the software (which btw you had to pay yearly or monthly for), it was a hassle, trying to keep your games in sync.
Not to mention having your system hardware capable of running multiple instances of the game, having enough resources and graphics capability to do so, even having multiple monitors to help with it.
I spent more time getting setup, keeping in sync, dealing with gear on multiple accounts than the time I could have spent solo leveling one.
I never renewed the MB software, and would sometimes just run two instances (or more of the game) and simply alt-tab between them. That was actually faster.
And while battle.net allows multiple instances of itself, that is actually useless for MBing. Because with the normal in-game launcher, you cannot launch more than one instance of the game, despite having multiple launchers.
Instead I have custom shortcuts (made to bypass the launcher entirely) that allow you run as many instances of the game as you want, all from the same game folder:
I haven’t bothered with multi boxing for at least the past 3 seasons. Just haven’t had desire or time to deal with it much anymore. But that video will show you how it works. The info is a bit old, but nothing has changed, even for Windows 10.
It shows how the shortcuts are made, and that you can run either 64 bit or 32 bit instances. (32 may be better for those with 16GB or less memory)