Welcome to the club . New to multiboxing myself, but there is a lot of support to be found and it's a lot of fun once you've got the setup down and get to harness the power of an entire team.

This video helped me a lot with my round robin Polymorph, Frost Nova, etc: https://youtu.be/q-ADHcJky2k?t=2213

I know there are some others out there as well... perhaps more in-depth even. Also I would check out the dual-boxing Discord for help if you are still having issues getting it to work. People in there have been very good with pointers and you can probably find someone running your comp specifically.

I'm running Pal tank, 3x mage, and Pal healer myself and AOE threat hasn't been much of an issue since I got salvation. Generally I would say the Warrior takes more time to master and use effectively for AOE threat, but those who are running a Warrior tank have some big advantages. From the videos I've seen they often take a little longer on the setup and look to get a few shouts off before they start DPS. Then with Frost Nova holding the mobs in place they should be dead by the time you pull aggro, or you can just nova again with a different char.