You can get 1 single guy to 70 faster solo than dualboxing.

Having said that, you can dualbox 2 to 70 faster than leveling 2 solo.

so it just depends on what your goals are.

Leveling while dualboxing is Not faster but you get 2 for that little extra time.

Hope this made sense.

Edit:
As to the How & why,,, Kill Quests are faster,, 1 for 1 drop rate collection quests are faster. Bosses & elites are do-able with dualboxing that you might not be able to do alone solo.

Its just not faster than leveling 1 guy (For me it was about 1.5 times the overall rate)