This topic has been brought up many times over the years.
Funcom has told players (through GM tells in-game or through email petitions) that it is ok to Multibox as long as no unattended gameplay occurs. The player must always be at the computer imputing commands when the characters are doing stuff. A key broadcaster is allowed to mirror a key between multiple clients.
What you aren’t allowed to do is automate gameplay. I.E. press one key and sit back and watch or go afk as your characters perform a string of actions.
If you want an answer from an Official source, don’t use these forums. Make a petition in-game, or go to https://help.funcom.com.
That said, as others have brought up, multiboxing will be quite difficult for a lot of the content in this game. Good luck though.