The spareMX uses the same physical address as the primary.
When the spare is Passiveit does not send any frames on the LAN whatsoever. Once the spare becomes Master it then uses the MAC of the Primary.
Here's some pcaps to illustrate:
Primary as Master, sending VRRP:
And the Spare, now as Master, sending VRRP:
The only way you can actually tell the difference it looking at the Priority value of the VRRP packets to know which device the it came from. I assure you though, these were sent from two different MX's that were in a Warm Spare.