The Joint Chiefs have no say in this. The CIC is the ultimate authority. If a national emergency is declared they can either obey orders or resign. To disobey would be a political move. That is against the UCMJ. The military can only disobey an illegal order. Nothing about this is illegal. Congress has already authorized the wall, they just want to play politics. The military is not about to get in a food fight with Trump. Most of them agree with him anyway. Even if they don’t most will obey orders.
I know they have no say. That’s why I called it a coup.