Here is the link to the Cisco bug document that mentions to be able to create a backup tunnel leaving through a backup VPN interface on FDM (not managed by FMC), the minimum release that supports this feature is 7.1.0.
https://bst.cloudapps.cisco.com...
Apparently, this is an IOS version related thing. I don't know where exactly the line is but older Cisco IOS were not sending "route originator" field as part of an EIGRP route update. This means, they would accept each others advertised routes even ...