I havnt paired a Paladin/Shadow Priest combo, but I am very familiar with both classes. Im sure Ill be hollered at for this, but if I were you Id roll the Priest in a Smite spec with the Paladin rather than Shadow. As Im sure you remember the finesse it took to level as a SP, between dotting and flaying, you'd probably be better off just loading up with a Holy Fire and spamming smite until dead.

/equipitem flamesuite