Multiboxing is generally allowed by MMORPG End User License Agreements, because the characters are still subject to all the normal rules of the game world and are controlled by the player directly. This is in contrast to bots that partially or fully control the characters, which are against the terms of service of most online games.[3]
Most MMORPGs only allow a single character per account to be logged in at once, so multi-boxers need a separate account for each character they want to play simultaneously. In subscription based services, this means paying multiple monthly fees, and buying several copies of game expansions. However, with trial accounts, multiboxing for free is possible. Using World of Warcraft: Starter Edition is one example which has unlimited play for free, albeit with certain in-game limitations surrounding maximum level, currency accrual, etc.[4]
Most game developers allow multiboxing in their games. To date, Age of Conan, Aion, Anarchy Online, City of Heroes, City of Villains, Dungeons and Dragons Online, EVE Online, Lord of the Rings Online, Heroes of Newerth,[citation needed] EverQuest, EverQuest II,
Lineage, Lineage II, Ultima Online, Vanguard: Saga of Heroes,Rakion Latin-Internacional, Warhammer Online and World of Warcraft all allow multiboxing. The Chronicles of Spellborn and Rift allow hardware multiboxing, but prohibit software multiboxing. Game publishers do not provide technical support for multiboxing, so while it may be allowed, you may find certain games are not designed to be multiboxed.[5] [6]
However, not all games condone multiboxing. Generally, a rule states that a player can only have one account from an IP address logged on at once. This is usually implemented to keep people from trading items that are limited per day to themselves. Games that do not allow a player to have more than one character at a time include Immortal Night and RuneScape.
Source
http://en.wikipedia.org/wiki/Multi-boxing