Either set up a round robin in your key broadcaster, or a round robin macro.
It's not possible in the way you are asking, but in practice, you can get it to work that way with the following cast sequences (assuming 5 characters here, set the reset to whatever the cooldown is for you).

Main:
/castsequence reset=60 Hammer of Justice,,,,
Slave1:
/castsequence reset=60 ,Hammer of Justice,,,
Slave2:
/castsequence reset=60 ,,Hammer of Justice,,
Slave3:
/castsequence reset=60 ,,,Hammer of Justice,
Slave4:
/castsequence reset=60 ,,,,Hammer of Justice