The default configuration in Ubuntu confirms that the 1Gps SFP and the WAN port share resources. They are bonded together and failover. I tested this with both, copper 1Gbps, and Fiber 1Gbps and it works as suggested.
eth0 is the fiber SFP, eth2 is the copper 1000baseT WAN
# bonding eth0 & eth2 with active-backup mode
iface eth0 inet manual
iface eth2 inet manual