Use action target groups.

Have an ATG for your 2 mages, name it like "2 mages". Then have it round robin the target group "2 mages". Make a second ATG called "Druids". Make the action for that as well. Now, 6 will work with your mages, or it iwll work with your druids.

I'd also HIGHLY recommend you do not use the build in round robin method. Instead, make a multi step action, that sends to each mage in order, and resets to the first step, AFTER the first characters cool down is up. This method is better, because it makes it predictable who is going to do what in which order. If you use the built in round robin method, I believe it simply just starts and stops where you left off in the rotation, which makes it a bit of guess work to figure out who, is about to frost nova for instance.

You can set the reset timer on the advanced tab. To setup the round robin, just make X number of steps in the action. At the top, I'd recommend ticking the do not pass for X seconds, and set that at like .1

This way, if you're in a state of panic(or lag) you can mash the button and not blow all your frost novas at once.