I'll assume you're using a software broadcast program... Keyclone is the easiest to set up, Innerspace is really tempting me to experiment with it (both are either pay or subscription options). Octopus, Auto HotKey, HotkeyNet are free (as far as I know) options.

If you press the space bar on your main screen, does the slave jump as well? Basically I'm asking if you are successfully broadcasting from your main client to your slaves/followers.

If this is the case, then you need macro's that work.

The easiest assist method to start with is Focus based.
I'd recommend moving from focus to something more powerful down the road.
Still Focus will get you started.

Assuming you have two toons, called Jack and Jill.
Both toons have a macro (on say Keybind Control F1):
/Focus Jack
/follow Jack

Both toons have a macro (keybound to Control F2):
/Focus Jill
/Follow Jill

Within Keyclone's maximizer settings, you have Jack's region hotkeyed to Control F1. Similarly hotkey the switch to Jill's region to Control F2.

When you press Control F2, keyclone brings Jills region to the front, and warcraft has both toons set Jill as their focus, and Jack starts to follow Jill. Control F1, reverses this.

Now on hotkey 1 - Jack is a warlock so he has:
/assist Focus
/cast Shadowbolt

Also on hotkey 1 - Jill is a mage, so she has:
/assist Focus
/cast Fireball

When you press 1, both toons assist the active (Focus) toon and then cast their spell.

That's a basic set up.