Re: Config a pair of Active/Backup interfaces

 You can try redundant interface, as sample config is below:

set interface redundant1 zone untrust
set interface redundant1 ip 210.1.1.1/24
set interface ethernet1/1 group redundant1
set interface ethernet1/2 group redundant1

You can move existing interface config to redundant one, you need to configure the subinterfaces manually, as below:

set interface redundant1.1 tag ..

You can change the primary status assignments via the CLI command set interface redundant1 primary interface1/1

http://www.juniper.net/documentation/software/screenos/screenos6.3.0/630_ce_HA.pdf for more details.

Thanks,

Vikas