I'd recommend the addon Jamba, found in the macro/addon forum of this site.
You can use a few different programs for your key broadcasting, Octopus and Keyclone seem to be the most popular. I'd recommend Keyclone between the two, but it costs $20.

The wiki and stickies are a great place to start.

You'll need to decide upon an assist method for the slave account.
Your basic options are:
-- Assist by toon name
-- Assist your focus
-- Assist party leader
-- The FTL assist system

The first is simplest, the last is most complex in terms of setting up, but also its more powerful once set up.

If you always run toon A as leader and toon B as slave, and never switch which toon is in the active window, then Assist by name is a decent option.
If you will switch which window is the active picture in picture, then you'll want one of the latter three assist methods.

Assist by focus, means all toons make the new active toon their focus.
Spells/Abilities are like this /cast [target=focustarget] spell name.

Assist by party leader uses paging, to keep focus free.
Your syntax is /cast [target=party1target] spell name.
For this system, your leader switch macro will put the new leader on page 2 (action bars, like Shift 2 in wow) and the slaves on page 1.
Page 2 has the same spells/macro's as page 1, but without assist. Page 1 has each macro with assist party 1.
So the master casts normally and the slaves assist the leader.

The FTL system is a serious pain to set up. But if you can get it working (and there's good instructions/explanations in its thread), it is the most powerful assist method.
It uses keystrings in keyclone, where you assist the active toon, based on which toon is active.
As in you just cast your spells normally, and keyclone tells the other windows to assist your main window toon.
A lot of set up to get it working, but this is the ideal system once set up, as you have focus free and don't need to mess around with paging.



Your macro's will use a combination of /cast, /castsequence and include the "," in sequences for pauses.
There are a ton of macro's people use to great effect, regardless of their assist method.

Using the same class for each toon is simplest as far as macro's go.
Using a more diverse mix of classes makes the set up harder, but gives you more flexibility.
Half the challenge, and fun, is setting up the system to work well in game.