1. Just what notes says: no need for 2 installations but they have a few minor advantages.

2. Fastest leveling is probably leveling a dps and a heal class together (e.g. mage & priest) and use the high level char just to kill some instance bosses for the loot. Another advantage is that you end up with two high level characters and you can decide which one you like to raid/farm/craft etc. with.

3.
a) Find out for yourself what kind of mobs get you the most exp. Some kill mobs that are a few levels higher than you while others (like me) like killing lower level mobs at better speed. perhaps it's best to get some addon that measures exp/hour and cash per hour and just try out which suits you better.
b) Don't be discouraged when the first levels are rough and you wipe a couple of times - it takes time to get used to playing two (or more) characters.
c) Remember: the ultimate goal is having fun! If you don't like playing a certain character or area or whatever just don't do it because someone theorycrafted that it's best.

Enjoy the dualboxing experience,
Bunny