your problem is indeed very annoying and common to xDSL backup solutions. Cisco has a 'new' feature called "object tracking" which is designed to address this issue, check this link:
Reliable Static Routing Backup Using Object Tracking
Basically, you track the object (ip address) on the internet via your primary link, if the primary link fails, the router will activate the backup link, effectively eliminating the dependency on the interface status...
HTH,
GP
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