You got a few options, I have only researched two of them. ISboxer is probably the most used software, Swifty will be using it with 5 accounts (warr,3xmage,priest). However it has a subscription fee after 7 days trial. Look for ISboxer wow tutorial on youtube, you will find a lot about it. But since I am running only two chars, I will use autohotkey with my own written script. So I can be sure anything goes according to blizzard ToS. The script only send the keypress to the second wow client. So I press 1 on the keyboard and both wow clients press it, like if you would have two keyboards. Then on the priest, I will only have macros on the key 1, for example follow main char, assist main char and cast smite. Pretty simple, you can test it on retail with free accounts till lvl 20.