It has been mentioned to me several times that if I am playing a class that can #twoweapon, then I should most definitely #twoweapon. At some point. In the future. However, I am not entirely clear on when this "future" is. Should I wait until I reach Expert in one or more weapon skills or should I wait until I can comfortably kill nearly everything I encounter and have a substantial supply of food (and therefore farm some earlier levels for weapon experience)?
#twoweapon, you want to choose which weapons will be your main and off-hand weapons before you start
#twoweaponing because when you are wielding both weapons, you only train your
#twoweapon skill. The wiki page suggests that you train your main weapon to the same skill level as your max
Given this information, I would suggest that once you have your intended main and off-hand weapons trained to those skill levels and you have enough AC without your shield to easily withstand hits from monsters at your level, that is when you should switch to
#twoweaponing. When you switch to
#twoweaponing you won't lose much damage per turn, so you should really switch as early as you can get the most out of it.