By casting it twice due to the upgrade , it is two separate actions, so you are doing the (0) action twice. It is refering to "them" in the action as two shards. So in resolving, you perform the action once and remove two shards and place one mindless zombie. Then repeat. The way it is written, it is only designed to be done once so you would only place two shards on an action and remove two at the completion of that action at the end of the turn. So at the end of the turn, you have to complete two separate actions.