LogFAQs > #980269220

This is assuming that the players themselves aren't hostile and nothing is personal about it; that's an entirely different issue.

I'm talking about when players decide to make characters who simply don't like each other and/or don't want to work together, to the point of open hostility. Both players understand that it's strictly a roleplaying thing and can actually laugh about it, but how would you feel as another player at the table or the GM?

Personally I think it's fine as long as it doesn't become something that hinders the game experience or becomes actively personal outside of the game. If I start to feel like the players were starting to resent each other I'd tell them to shut it down, and if their rivalry/hostilities became something that takes too much focus away from the other players at the table or derails multiple scenes or combats, I'd have them tone it down a bit.