Must be something in the Victoria air! To expand a bit:
First off, I think it somewhat depends on what edition/game you're playing. It's way easier to run multiple characters in 1e than in 3e, for example (I know, having done both). That said:
As a DM I like it when there are more PCs. I find it's much harder to balance encounters when there are too few characters in the party. In small groups I think it's necessary to have at least one player double up on PCs, and the rest of the time I don't find it detracts from the game. Net positive.
As a player I enjoy playing more than one PC, and I like it fine when other players do it.
Another consideration is that if you as player are running more than one it's not the end of your night if one gets captured or killed or whatever; this opens up some options for the DM.
Further, the larger the party is, the more opportunities there are for in-party interactions (romances, conflicts, murder, etc.) and that's always a benefit.
1) Make one, if not both, characters mechanically simple. Given the choice between a battlerager and a greatweapon fighter, choose the greatweapon because there's less involved at the table.
Also, try to make them take different roles (to swipe a 4e term) within the party - I usually prefer to have one front-line tank and one back-line healer or wizard.
The suggestion upthread about banning inter-relations between characters run by the same player is sound most of the time; however, there's occasions when someone has an idea going in for two characters to be related somehow and it's usually not the end of the world to allow such. More fun is when one player's characters come in conflict with each other...
And having more than one character in the world (as opposed to in a given party) goes without saying.
Lanefan