It is forbidden to multibox more than 2 accounts (2 characters) within the same map zone (Elwynn Forest, Wetlands, Mulgore, The Barrens are all examples of map zones). There are no limits on account/character count in Capital Cities.
Can I manually control two windows (e.g., using the in-game /follow feature) in the same zone to level up a second character, provided I do not use any third-party software or automation?