So after looking a little on wowhead was still curious, to get 40/40 do u still have to have a toon from each faction to complete it? bit meh if so altho im sure it wont be hard to find someone its still fairly annoying if u have to >.>
You do not. I completed this achievement on a single character on alliance.
If you are Horde (at least in my experience) you have to make a level 1 Alliance and go battle one Alliance tamer at low level to complete the achievement.