Multi-boxing describes a single player using two different browser tabs to control two different cells. Basically, they are teaming with themselves. Multi-boxers use two different tabs and Alt+Tab or Ctrl+Tab to alternate between them. Browser extensions are usually used to assist as well. Multiple monitor or computer setups may also be used to muti-box.
Multi-boxing is common in many other types of games, especially massively multiplayer online role-playing games (MMORPGs). See the wikipedia article for more information about Multi-boxing.