Asonimie has the targeting of Wind Shear, by Arena Frames in his big Shaman post.
Pretty much, preface each shammies macro with a Wind Shear.
Have Shammy 1 target Arena 1, Shammy 2 target Arena 2, etc.
Which gets you a good chance to occasionally interfere with the opposition.

Since macros execute top-down, assuming you have space in the macro...

Have Shammy 1 preface their macros with:
/cast [target=arena1] Wind Shear
/cast [target=arena2] Wind Shear
/cast [target=arena3] Wind Shear

Have Shammy 2 preface their macros with:
/cast [target=arena2] Wind Shear
/cast [target=arena3] Wind Shear
/cast [target=arena4] Wind Shear

Since Wind Shear is off of the GCD, you could adapt this for IS Boxer.
Create a keymap for each shammy.
The keymap is two steps.
In step 1, have your Wind Shear macro; it can be five lines long, with a different arena# for each line.
Only the first valid target will execute.

Each shammy has their own keymap, because you give each a different priority arena frame.
So theoretically, if 3 opponents who cast are in range, and Wind Shear is available on all shammies, each opponent gets hit once.

In Step 2, put whatever your normal FTL DPS key is.

This is the same idea as weaving IWT into a melee macro.
I'd personally set the keymap to advance on "key press or release", instead of the default "on key release only".
But if you are one of those who don't want to take advantage of something Blizzard permits (see Keyboard API), it is easy enough to just spam each of your keys twice, or your click macro twice as quickly.