Players with Multiple Characters (DMG)

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Each player usually controls one character, but sometimes players may want or need more. Multiple player characters are fine in the right situation.
Once your campaign is underway and players learn more about the game world, they may want to have characters in several widely scattered areas throughout that world. Having multiple characters who live and adventure in different regions allows a lot of variety in the game. The characters usually are spread far enough apart so that events in one region don't affect what transpires in the other.
Sometimes players want to try a different class or race of character but do not want to abandon their older, more experienced heroes. Again, spreading these characters out across the world is an effective means of keeping them separate and unique.
Whenever possible, avoid letting players have more than one character in the same area. If, for some reason, players must have more than one character in an area, make sure that the characters are of significantly different experience levels. Even this difference should keep them from crossing paths very often.
If multiple player characters are allowed, each character should be distinct and different. It is perfectly fair to rule that multiple characters controlled by one person must be different classes—perhaps even different races. This helps the player keep them separate in his imagination.
If a player has more than one character available, ask him to choose which character he wants to use for the adventure—before he knows what the adventure is about. If a single adventure stretches across several playing sessions, the same character should be used throughout. All of the player's other characters are considered busy with something else during this time.
Avoid letting players take more than one character along on a single adventure. This usually comes up when the group of characters assembled for the planned adventure is too small to undertake it safely. The best solution to this problem is to adjust the adventure, use a different adventure entirely, or supplement the party with NPC hirelings.
==Multiple Character Problems==
Playing the role of a single character in depth is more than enough work for one person. Adding a second character usually means that both become lists of numbers rather than personalities.
===Shared Items===
One single player/multiple character problem that needs to be nipped in the bud is that of shared equipment. Some players will trade magical items, treasure, maps, and gear back and forth among their characters.
For example, when Phaedre goes adventuring she takes along Bertramm's ring of invisibility. Bertramm, in exchange, gets the use of Phaedre's boots of speed. In short, each character has the accumulated treasure of two adventurers to draw on.
Do not allow this! Even though one player controls both characters, those characters are not clones. Their equipment and treasure is extremely valuable. Would Phaedre loan her boots to a character controlled by another player? How about an NPC? Probably not, on both counts. Unless the character is (foolishly) generous in all aspects of his personality, you have every right (some might call it a duty) to disallow this sort of behavior.
===Shared Information===
Information is a much more difficult problem. Your players must understand the distinction between what they know as players and what their characters know. Your players have read the rules and shared stories about each other's games. They've torn out their hair as the entire party of adventurers was turned into lawn ornaments by the medusa who lives beyond the black gateway. That is all player information. No other characters know what happened to that group, except this: they went through the black gateway and never returned.
The problem of player knowledge/character knowledge is always present, but it is much worse when players control more than one character in the same region. It takes good players to ignore information their characters have no way of knowing, especially if it concerns something dangerous. The best solution is to avoid the situation. If it comes up and players seem to be taking advantage of knowledge they shouldn't have, you can discourage them by changing things a bit. Still, prevention is the best cure.
And remember, when problems arise (which they will), don't give up or give in. Instead, look for ways to turn the problem into an adventure.