Lets take a more complicated case.
Each MX has two uplinks, and each MX has a cellular link.
If both uplinks of the primary MX are down and any uplink on the spare is up it will failover to the spare.
If both uplinks of the primary MX are down and both uplinks of the spare are down, and the celluar is working on the primary it will use the cellular on the primary.
If both uplinks of the primary MX are down and both uplinks of the spare are down, and the celluar is down on the primary it will use the cellular on the spare.
This is not configurable.
https://documentation.meraki.com/MX/Deployment_Guides/MX_Warm_Spare_-_High_Availability_Pair#Cellula...