It will probably take some "toying" around with to get it where you like it.

I have my paladin down to 3-4 main tanking button/macro combos.
I'm looking into doing the same with a DK, but they just have so many spells/abilities it's going to take me a while to figure out how I want to use them.
Right now I have "3" set for every slave's spam-DPS castsequence. Then I use "Shift+3" for a smaller/quicker version to take out trash mobs. (no sense putting a 18 second DoT on a mob that will die in 5 seconds). Keep in mind, if you are spamming the DPS key, it doesn't really matter if Slave 1 casts a 3 second spell, and Slave 2 casts an instant spell...just keep spamming the button and their individual castsequences will continue as soon as they can.

You might want to consider making one of the Shaman Resto, but I'm sure others can bring up points against this. It mostly depends on your play style.
But, by doing this, you can let your other Shaman continue DPS'ing while one is used to heal.