Because their camera angles are different, even if minutely so.
And because characters will be slightly different in their facing...

There isn't really a method where they will stay in sync with each other, over distance.
The more you move them, the more variations in their movement will crop up.
Over a short distance, you won't notice much...
But if you ever try to move a group in a formation, you notice it breaks apart fairly quickly.

The only way to reliably move them together, is to use follow.

For a boss mechanic, I'd mimic the "Others Jump" mapped key.
Which sends SPACEBAR to all the windows, except the current character.
You could use the WASD keys for the main toon/current toon, and the Arrow Keys for the others/slave toons.

If you're not using IS Boxer, you'd likely have to play around with Keybinds.
If you almost always lead with the tank, then the slaves would consistently be the same toons.
And this wouldn't be much of an issue.
Rather than using left arrow/right arrow to turn, for a boss encounter you'd be better off with strafe (same as Q and E, for the main).
You could alternatively go Shift + Q, Shift + W, etc... whatever works best for you.

When a puddle of goo appears.
Everyone strafes to the left/right, to get out of it.
When they have to move again...
Same deal, to a relatively clear space.
With a few moves here and there, they'll be in relative sync.
Once sync breaks, your only real option to reestablish it, will be to use Follow.

If you're doing this within IS Boxer.
You could make a mapped key... probably a Warcraft Macro Action, that is: /follow and only for the current window.
Then make another mapped key, which has only the toons you want to re-sync, each target only one of them (the same on each).
And then have those windows each run the previously made wow macro action, to follow that one toon.
Move away from where you want them to face, just far enough that each will follow... then turn about and run through them, so they face the correct way... then back up marginally, so the toon is once more stacked with the others.