I wrote an addon called Multiboxer about a year ago that sounds like it could do what you want. That in combination with your macros could probably do it.

It works by changing leader of the group, and then swapping the active bars, so you can have a bar setup with /assist party1 on all the "assist bars", and then your normal spells on the other bar.

There is no GUI and it takes a bit of work to get it going, but I still use it even in it's simplistic form from when I wrote it a year ago.

I use buttons F1 through F5 to change screens via autohotkey which simutaenously presses a button which activates the addon.

So I press F1 and toon1 gets action bar 1, toons 2 through 5 get action bar 2 (or vice versa, can't remember), and toon 1 is also promoted to leader.

I press F2 and toon 2 gets action bar 1, toon 1 and 3 through 5 get action bar 2 and toon 2 is promoted to leader.

ect...

I don't have to worry about any focuses or anything with this addon, although they could make it more powerful. If you do a search on google for "multiboxer", I believe it's the first result - it links you to curses website where my addon is located.