As a right-hander, I always felt that guitar-playing demanded a lot of left-hand dexterity, and that learning forced me to develop my left hand skills.

I learned to type at the same time that I was learning guitar, and to this day my left pinky does a much better job on the QWERTY keyboard than does my right pinky (which I don't use at all). I make most of my typos with the right ring finger, on the O, P, L, :, ., and / keys.

Lefties can use any of the three options mentioned by Kevin, above, and I suppose you could find at least one left-handed string player to endorse each approach. I think that playing the standard-strung instrument upside-down-and-backwards would be the most difficult and counterintuitive approach, but at least two very notable (and very different) players have used that technique very successfully: Libba Cotton and Jimi Hendrix.

If it were my kids, I encourage them to both try the standard "right-handed" method, at least at first. If the lefty twin exhibited any problem at all (including simple lack of interest compared to the otehr twin), then I'd experiment with the other methods.