If you plan to level through the LFG group primarily, it may be a very good idea for one of the two toons to be a tank because it reduces the waiting time considerably according to my stepchildren.

One thing to keep in mind when leveling a duo with no healing spell is you can learn pretty powerful bandages at relatively low levels. For example, you can learn Frostweave bandages at level 35 should you teleport to Dalaran. A DPS can apply the bandage while in combat to get the tank back to full. We also found it is very useful for a healer to apply bandages in combat when mana conservation is an issue (this is true of some encounters when undermanning levelling content). The several seconds out of the 5 seconds rule, combined with the power of the bandage, can extend the healer’s mana by a fair amount.

Of all the above combination, the protection warrior and restoration druid is my favorite. The HoTs are typically powerful enough for me to DPS (as cat or caster) or OT a caster before they need to be reapplied. I like it because it provides variety and room for experimentation.