Sounds like you need to upgrade to ScreenOS 6.3 version. Prior versions only supported have one proxy-id pair. With ScreenOS 6.3 you can have multiple pairs at the same time.
Be sure to verify that you have the new ScreenOS signing key on your device BEFORE the upgrade. If you have the old key and attempt the upgrade the device can fail to boot anymore.
https://kb.juniper.net/InfoCenter/index?page=conte