Mmmmm. Keldorn & Minsc shouldn't attack each other since they are both good aligned characters. It is possible that one of them is under the influence of the Charm, Dire Charm or Domination spell (placing them under the control of an enemy spellcaster) or a confusion spell which causes them to do random things which includes attacking their allies.
The best way to find out which is the case is to look at the portraits on the right of the screen next time this happens - they will have little symbols on the portraits. To find out what the symbols mean, either refer to the 'quick-reference' card which came with the game, or look at the Character record screen which will tell you if your characters are under the influence of any spells whether good or bad.
There are several ways to protect against mind control spells - some items have 'mind shield' (such as the Helmet of Charm Protection) and the spell Chaotic commands protects against confusion. If you have none of these, when it happens get your character with highest AC and HP to stand next to the affected character and take all the damage, or move the rest of the party out of the affected character's field of vision until the spell wears off.
------------------
Time is an illusion. Lunchtime doubly so.
Epona of The Laughing Hyenas