My question involves a child custody case from the State of: Arizona

My husband is in the military and deploying soon. That has made us think about making a Will for our son in case either of us passes. My husband has another son with his ex, and he would like to write up something for custody of him in case the unthinkable happens. I know in general if he passes, his will automatically get the child. But what would happen if they were to both pass and the two if them both have a Will designating 2 separate people as a guardian of their child?