The best advice I can think of is to try them both before you go, then stick with whichever one feels the best. I'm not really sure there's a "right" answer to your question--if it works, it's good!
I used one trekking pole, but my knees are generally strong and don't cause me trouble, so I don't really find two poles necessary. Some people say they feel "lopsided" without the symmetry of a pole in both hands. And if you have knee problems, the added support of two poles might help.
When I hiked the AT, I started with two poles thinking that's what "everyone" used and it was necessary. When it started raining, I put one away and started using an umbrella which is when I decided that I liked having the one hand free for other uses. Eventually, I just gave away my second pole. So if you aren't sure which fits your style better, you can always start with two and go down to one if that suits you better. It's a lot harder to find a good mate for one pole if you later decide you're a two-pole person, though. =)
-- Ryan