if i dont have the Happy NPCs part of EOU installed, is it possible to say have both Keldorn and Viconia or Minsc and Edwin in your party for the entire game (including TOB)?

or will one kill the other or leave?

No, it's not possible. There will be conflicts and some kind of parting :)

I heard that battling appears randomly and quite infrequently. If two NPCs start a battle, reload a previous save and chances are high that it will not happen for some time.

Once in ToB they don't fight to death AFAIK. (but you can expect the common bickering)

hmm guess ill have to see how it goes then.

They will eventually go at each other, and try to kill each other. But as Silverstar said, the triggering of the talks where they do it is random, so you can put it off for a while by reloading when they chat. I've got it documented, but apparently haven't submitted it yet. Weird.

Not having the banteraccelrator gives you more breathingspace I'd presume :)