When placing two lines of backer rod and sealant at window and door jambs with an air / water barrier taped to the jamb flashing where is the main line of defense?

Main line of defense is the second line of sealant in line with the air / water barrier using a breathable backer rod to allow sealant to cure.

Outboard sealant protects main sealant from the environment and acts as a first line of defense in line with the exterior face of glass and not block weeps.