Shroud of Avatar has posted an official message clarifying the team’s stance on multi-boxing.
In short, multi-boxing is allowed. What isn’t allowed, however, is having multiple clients enabled in the background,
“The only thing that is not allowed is having a second (or third+) client in the background that is executing some automatically triggered skill like autoattack or crafting. For example, if you run multiple clients and switch to a second one to cast a heal spell, that is fine BUT your other account might miss an auto attack or two while you do that. If you are a streamer running a single client and want to tab over to twitch to chat, that is fine and nothing changes as this will only impact people running multiple clients.”
Additionally, if you are indeed multi-boxing, auto attacking in the background is not ok. You’ll effectively be blocked by the client. If you’re running a single client, don’t worry. This won’t impact you. You can check out the full post here.