Hello
The following KB articles talk about the scenario that you have explained. Hope they are useful for you.
How to configure dual ISP links which act as primary and back up on ScreenOS firewalls?
https://kb.juniper.net/InfoCenter/index?page=content&id=KB20550
How to configure Interface Failover by using Track-IP on ScreenOS devices
https://kb.juniper.net/InfoCenter/index?page=content&id=KB7432
Regards
Vatsa