Assuming he's a minor, and you own both accounts (your name), you are fine to use both.

But, if you're both getting into dual boxing, as in 4+ accounts, I'd consider running 5 total accounts across two PC's.
That gets you: Tank + Healer; and him: DPS x3.
You'd basically be able to do all PvE content; depending on your tank/heal choice, you could pvp a lot too.

As far as heals go...

#show Name of Heal Spell
/assist Tank'sName
/cast [help] Heal; [help, Target=TargetTarget] Heal; [help, Target=TargetTargetTarget] Heal

That will work quite well.
The 3rd (last) condition will only be necessary in pvp.
If you stick to mobs (pve), the first two will be all you need.
You can target with the tank, and heal a friendly.
Or if the tank targets a hostile, then the heal goes to the hostiles target, which is usually the tank.

You could make 2-3 keys (or 5), to cover most healing situations.

Druid - Lifebloom, Regrowth, Rejuvenation, Wild Growth
Paladin - Cleanse, Flash of Light, Holy Light
Priest - Renew, Shield, Flash, Greater/Penance
Shaman - Earth Shield, Chain Heal, Healing Wave, Lesser Healing Wave, Riptide

That's basically what I use, although I'm using IS Boxer and repeater regions, rather the assist macros, although I have some for out of party heals too.