If you want speed you will be quicker levelling them solo.

With just two you get half your normal XP per mob, and while you can kill a bit faster and with a bit more safety you aren't going to be able to kill twice as fast. nor can you grind through instances until its not worth doing so. Either that or find some others to play with, so at least you can grind instances and or get the group bonus.

Personally I like levelling more than end game so two while slower is still quite a bit of fun.