Most any two classes can successfully duo together and can take on the majority of the available content.
My suggestion would be to include a healer if possible. Most any tank or DPS class would do to pull and tank the mobs and the healer would do double duty as medic and additional DPS.
Your brig (a DPS class) would pair up well with a druid (warden or fury) either of whom can provide excellent DPS as well as a variety of heals.
However, a better choice than the brig might be a bruiser. This character is one of the tank classes (an evil monk) and so, when needed will be capable of taunting mobs off the healer and then holding aggro while the two of you carve up the mob.
If you go with Bruiser, you might also consider a defiler (evil shaman) as the healer since the shaman wards work well with the bruiser's avoidance.