There are two options to round-robin, within IS Boxer.

You can put your Shaman characters into an Action Target Group, called Shaman.
Making several relevant ATG's is a good idea.
I have one for Healers, another for Melee, another for DK's, another for toons that will do a Ground Click/Target spell on the Shift A keybind, etc...




You can do a warcraft macro action, or a keystroke action (that sends the keybind for Tremor Totem to warcraft), and use your Shaman ATG as the target.
Then just check the box that says to round-robin, within the target group.




You can also give your mapped key more than one Step.
A step is a different result on subsequent presses of the same key.
After the last step executes, it automatically moves back to the first step.

On each step, you would have one shaman with the macro action or keystroke action for Tremor Totem.
So four steps for four shaman.

You have a few extra options, such as limiting the step so no matter how you spam, it cannot advance to the next step for x.x seconds.
You can also check a box, so it will only trigger once... if the box is not checked, and you're on a GCD when you reach this step, that keybind might be sent several times, which can be a good thing.
For example, do not advance for at least 1.5 seconds, without the trigger once...
Now you can spam your Tremor Key, and once it drops stop spamming... and basically not drop 2-3 of your Tremors at once.

In the advanced tab, you have options like:
- Reset to Step 1, after x.x seconds:
---- From the First Press
---- From the Last/Most Recent Press
---- From when it advanced to Step 2.

You can get creative with that....
Round-robin an effect like Heroism, and auto reset to the first toon after enough seconds that Heroism is once more available.