Dual melee is harder, because follow breaks once melee starts.
If your slave toon is pretty much a full time healer and not a combatant, you won't have this issue so much.

Both toons will wear mail armor at this stage, which is good durability; your healer will probably go weapon + shield for added defense.
The paladin is an excellent single target healer.
In the 19's bracket, its generally rogues and hunters who are the twinks. Not sure how a twinked pally would do.

The paladin as a healer lacks ranged attacks, which won't give you as much support dps as another healer type.
At least not until they're both in melee, but at that point follow breaks which makes mobility an issue if the battle moves.

If you want to make it work, it will work.
Generally only one melee is easier.