As an American, I default to eating with primarily one hand, but I don't ascribe any particular value or reason to it. It's just the way I'm used to doing it. Sometimes I try to use two hands (like a knife in my right cutting a piece of steak, and a fork in my left spearing the bite to eat) but I find it awkward just because I'm not used to it, and if I don't concentrate I fall back on what I'm used to.
I wouldn't correct anyone for using two hands. You look greedy if you are shoveling food into your mouth, making a mess, ignoring people in favor of eating, grabbing all the bread before others can have some, taking thirds before everyone has gotten firsts, commenting on the inadequacy of the food (if it's provided by someone else), etc.. But I don't think someone sitting there, conversing with others and moving neatly at a moderate speed would look greedy, whether they are using one hand or two.