I'd recommend the "No Focus Manifesto" as your assist method.
Go with the Jamba addon, that makes boxing so much better.

Basically, with two toons, this is very simple.
The no-focus-manifesto will be more useful with 3 or more toons.

Essentially, you have a targeting order, where each living toon targets others in the order you specify.
With only two toons, you'd just have each toon target the pally and follow the pally.

With the priest as your healer, it's probably easiest to just assist heal.
Assuming you go with a hotkey on each to target and follow the paladin...
The priest will need something like this (and a version for other heals like Flash Heal, Greater Heal/Penance etc):
/cast [target=targettarget,help] Renew; [target=targettargettarget,help] Renew
Since the Priest is targeting the Paladin, you have them cast at the Paladin's target if that is friendly.
If that condition fails, then you try to cast at the target of the paladin's target, which is probably one of your toons.

The Paladin will eventually go with a "96969" macro (forum search for it) if you're going Protection.
If you're going Retribution, I'm not sure how your macro's will be, but the search feature is your friend still.

I'd either have the Pallies DPS key on two buttons...
And the priest have a DPS key only only one of those...
Could be something like:
/castsequence [target=targettarget] reset=target Shadow Word: Pain, Devouring Plague, Holy Fire, Mind Blast, Smite
Or whatever you desire. You could go with a more complicated macro using a /Click Castsequence.

Alternatively, put the priest DPS on the same key as the Pally (who has only one DPS key), but include a modifier on the priest.
That way the priest only DPS's as well, if the mod key is pressed.
/castsequence [mod:alt, target=targettarget] reset=target Shadow Word: Pain, Devouring Plague, Holy Fire, Mind Blast, Smite