If you're not planning to take them into 5 man's then there's not a huge need for a dedicated tank class on the other hand it can't hurt having one either. Most of the non-instanced group quests can be managed with an off spec tank class with a marginal tanking set.

2 x Hunters, or a hunter + healing capable class would provide a lot of versatility and allow you to work on some of the instanced content.

One thing to think about is that in the future if you decide you want to try instanced content with your duo it is easier to find 3 dps to join your tank / healer duo than a tank / healer / dps group that's willing to give a duo-boxer a shot.

-Stav