cr:7.3.2d

All declared cards for an attack-target are put onto the active chain link as a single multi-event as defending cards for that attack-target. The order in which the declared cards become defending within the multi-event is determined by the order in which were declared.[1.9.2]

Example: Bastion of Unity has the text “Unity - When this defends together with a card from hand, this gets +1{d} until end of turn.” If one player declares Bastion of Unity and another player declares a card from hand, Bastion of Unity’s effect will trigger because both cards defend together as part of the same multi-event.