Basically, once auto attack starts, and for a melee toon it will as soon as you click any attack ability, auto follow breaks.
It is what makes boxing more then one melee toon more difficult then just using ranged classes.

If you wait until both toons are in melee range with the mob, before the slaves start to fight, that will solve the issue.
If the slave has the same attack key as the master toon, then pressing an attack on either will put them both into combat, which breaks follow.
If this is the case, either don't attack until they're both in range or give the slave a delay (comma) before the first attack... so you main tags the mob while your slave toon does nothing on the first keystroke.