With a two-box setup, both on the same computer, you're going the software route.
Hotkeynet works for many, I personally use Keyclone and recommend that; either will work for you.
I highly recommend Jamba addon, with the thread by Jafula in the macro/addon forum.

You're going to need to choose an assist method.
With two toons, you can have one as primary and one as secondary, then have the secondary always just Line1 /assist Primary and Line2 /cast Spell.
If you want to change which toon is primary so they can both be primary or secondary (more important as you add more toons to box), your basic options are Focus Based or Leadership based assisting.

With focus based, each toon has a focus key per toon. So macro 1 will read.. /Focus Shaman'sName and macro 2 will read.. /Focus Mage'sName.
Both toons will have both macro's and on the same keybind. So if you press the Focus Shaman macro key, they both click that macro.
Then all your spells or castsequences are either: Line1 /assist focus -and- Line2 /cast Spell --or-- /cast [target=focustarget] Spell.
The first will have your slave target the masters target and then cast. The second will not even target anything but it will still cast at the master's target.
The second is probably better for pvp, as you don't broadcast what you're targeting.
The first with target is good for pve, as your background toon can have a macro to targetlastenemy and then skin the target, while your main can herb/mine for all gathering skills covered.

With Leadership based assist, you're essentially taking advantage of paging.
You have assist macro's the same as focus, except they're two line macro's. Line1 /script PromoteToLeader("MageName") -and- Line2 /changeactionbar #.
On the mage's macro when the mage is becoming leader the # is 2, for action bar 2. Same thing for the shaman's macro when the shaman is becoming leader, this is also 2.
On macro's where any of the other toons are becoming leader, the # is 1, for action bar 1.
On bar 1, you have the same spells/abilities as bar 2, but they're all macro's where Line1 reads: /assist [target=party1] -and- Line2 is either /cast Spell or /castsequence Spell 1, Spell 2.
On bar 2, you have the same spells/abilities as bar 1, but they're straight spells or castsequences without any form of assist.
So whichever toon is lead casts normally and all the slaves assist that toon.
The reason for the paging, is the leader sees the 2nd toon in group (Press F2 on party leader) as the party1 toon. Everyone else (not the leader presses F2 [party1] and gets the party leader.