If you use ANYTHING to assist you in any way, that is against the rules for multiboxing. No hardware, no software, no home made keyboard tapping device sitting on your desk.
Do not do anything at all or use anything at all. You can manually switch between game instances yourself. You can use the follow command in game.
That is it. Having your team casting and performing actions in synch due to software or hardware that may assist or streamline it is likely to get you penalized.
Not really, no. The policy has been in effect since early 2021.