Currently,
- questing, levelling, farming mobs, playing pvp, gathering herbs/ores with alttab priest on follow is fine.
- what is forbidden is when the two characters perform actions simultaneously.
The mechanics behind this is that it takes some time for you to use Warrior and cast Charge, alt-tab to Priest, cast Power Word: Shield/Renew/Smite/whatever and alt-tab back to your Warrior to control him again. It becomes increasingly difficult to do this fast when you have to do complicated things like targeting two enemies and there is always some time lag. Even a very fast response will take usually 0.1 delay + the delay of your system activating the other window, possibly even drawing what's going on the screen + delay between keypresses and signal transferrence.
Controlling your other character via automated commands or cloned keypresses won't have any delay, so for example Charging and casting Smite will both happen at the same milisecond, which is nearly impossible for humans and even more impossible to do repeatedly.
If you do not trust someone, you don't like/can't record, the easiest way to check is to write /combatlog. This will create a Log folder in your WoW directory, which will contain a combatlog.txt file and that will include all actions your combatlog observed with miliseconds until you logged out from WoW.
