A player who wants to develop more than one character may do so, but there are a few rules to go with the decision.
No character interaction
Two characters belonging to the same player may not exchange items or services, the two must remain completely separate at all times. If you have a wizard and a fighter and the wizard finds a magic sword he cannot attempt to arrange for the sword to get to the fighter even through another player.
One character at a time
While a player may play different characters in different sessions they can never have more than 1 character in the same session even if one of them dies.
No shared experience
The characters must have the number of sessions tracked separately, so several sessions playing the fighter will not help the wizard advance and vice versa.
Replacement vs. Multi
A replacement character brought in for a retired character has looser limits and will start at the level of the current lowest level party member as if they just reached that level. If the lowest level member is 4th (multi class 3/3 or 2/2/2) the character begins at level 4 (3/3 or 2/2/2) and needs 4 more sessions to advance. Even though they begin at a higher level they start with no magic items and the regular 300 GP for beginning gear.
The replacement rule allows a player to retire a character without falling far behind the rest of the party. The retired character becomes an NPC and characters wishing to access their services will have to treat them as such.