You can configure lower RIP metric at interface level at both the ends on the backup link. Also, you can consider the track-ip option to bring down the primary interface in case of any issues. below are the references:
http://kb.juniper.net/InfoCenter/index?page=content&id=KB4166&smlogin=true&actp=search
http://www.juniper.net/techpubs/software/screenos/screenos6.3.0/630_ce_Routing.pdf
http://kb.juniper.net/InfoCenter/index?page=content&id=KB7432&actp=search
Thanks,
Vikas