What do you want to achieve with these two characters?

For simplicity it's good to box two of the same class. It used to be casters were easier to multibox, but recent changes mean that melee are just as viable.

When they are 80 what do you want to do with the chars? PVP? Play them together in PVE. Or do you want to play them solo, in which case having two seperate classes would be better.

Without knowing more about what you want these characters for it's difficult to make recommendations.

You can't really go wrong with 2 elemental shaman though. They can deal damage, have a load of situational abilities with totems, wear mail at higher levels so can take a hit or two. They can res. They can heal without dropping out of a form.