Our most successful boxer (Ellay) for competitive arena runs with 4x shaman played by himself and 1x healer played by someone else.
Shaman's are quite powerful as you add more of them; even if their dps is comparatively weak individually, they provide controlled burst, and with enough of them you are deadly.

In the 2's bracket, you won't have enough burst to compensate for your being one person while your opposition is two people.
I'm not saying you won't win some of your matches, but pretty much whatever you choose you will be at a disadvantage.
You can still run 2's and accumulate arena points, but don't expect to hit a high rating as a boxer in the 2's bracket.

Two shammies is a good choice, if you'll eventually be four shammies.
For BG and world pvp, a lot of four or five toon compositions are very strong.
Generally for arena, you'll want as few different classes as possible, or preferably to be playing all your toons as the same class.
Some people have done fairly well as 5x one-class, but 4x one-class plus someone else at the 5th, have consistently done much better.