IMO, you can spec any way that appeals to you with this duo.
Feral will of course bring great DPS and you'll be taking stuff down really, really fast. You can still heal as needed while questing, and heal instances with healing gear.
Speccing resto will make finding groups easy (you've got a tank and healer), and I really like the tank/healer combo for duoing myself - the two of you together will be invincible, taking down tons of mobs at once, and if you also do some casting DPS you'll blow through quests pretty fast. Plus, if you have any interest in leveling a healer in the healing spec, as one part of a duo is a great way to do it.
I have no personal experience with the balance spec, but I'm guessing that with a warrior, ranged DPS would be awfully nice, and again, you can still heal as needed.
All the duo characters I've ever done have been for casual fun on nights when we feel like doing something different. If that's the case for you, I'd say just pick something that's a refreshing change from whatever your main is and have fun with it.