Activision-Blizzard pulls in billions of dollars a year. They don’t allow multiboxing because of the pitiful amount of extra money it brings in.
You can multibox with multiple computers controlled by one set of input devices.
Also, the simplest dual-box set up would be to have a multi-seat flying mount. First account mounts up, second account hops on, you now have two characters that can farm without any software or hardware.