A recent forum thread asked the same question, which you may peruse for inspiration.
However, RotR is not so hard that you should run into problems with almost any 2-character group. I would only recommend that you have at least one character with the ability to make decent Wisdom checks (and, ideally, with Divine magic); consider Kyra, Seelah or Lini to be one of your two characters, and the other to be just about any character of your choice.
I'd also add you should try to have at least one character that can reliably have a good combat check. (Most characters are fine for this, but there's a few combos like Bard/Cleric that can have a bit of trouble). The other advice, which was brought up in the other thread, is it's more fun if the two characters have different niches, which will make more of the boons you pick up useful/exciting.