If anything, a third player should make a campaign more interesting. Think of it this way. Two points (characters) make a line. A line isn't very interesting, now is it. Add a third point, and you got yourself a full-fledged triangle! I'm not sure about you, but personally, triangle seem just a tad bit more interesting. Definitely make sure your players are comfortable with the new player, but I say that the more (to a certain extent) the merrier!
Woah, I like this example :-)
IMO, only two players are rather uninteresting. All groups I have played with, tried to have at least 3 players, for various reaons (one, that is more interesting and second, to solve "ties" in discussions, what to do now (if both players want to take different actions, but can't settle to one action, a third player can solve this with is "vote").
For the other part, with the player, who can only fight, and the player who can "only roleplay": I wouldn't give the "weaker" person a bonus, because it would be unfair against the stronger person. The fighter has an high attack skill, but the "weaker person" hits, because you give him an advantage, it would be rather demotivational for the fighter (vice versa for an "social bonus" for the fighter, because the "social person" don't get this bonus).
One thing, I would do: Don't play nice with them. If the "not-fighter" is showing some "danger potential" in a fight (he has drawn a weapon or shown some supernatural abilities), he will also be attacked, like his comrade. And because of his bad stats, he will get hit and will lose life points and will fear about his characters life. Also, don't let the "social character" make all the social rolls. Perhaps an NPC will hear the opinion of the fighter (because he looks stronger, and the NPC will hear an advice from someone who knows to fight). Also, sometimes seperate the characters. In this case, both can only be using their own skills, and both should have some decent values in some "standard abilities".
This all said: You should allow, that the characters have some distinctions, in which the one character is better and the other is worse. If an character is planed to be a sneaky, lock-picking theft, it wouldn't be okay, to have two such characters, because both thefts would only argue, who should pick this lock. But all characters could have some value in stealth (because if only one character has stealth, he couldn't often use it, because the other characters would ne left behind OR they would make enough noise, to wake everyone).