Same priority for masters in a CAN network
In the CAN context, there are no masters. You can only speak about transmitters and receivers. The priority is set only between frames ID during the arbitration phase.

Therefore if you transmit the same ID message, the first transmitter emitting is going to transmit its frame. If they transmit at the same time, the frame is going to be corrupted (as soon as a bit differs). The ECU are going to send the error frame and therefore the message is not going to be taken into account by the CAN controllers.