Explanation
"You can add secondary VNICs to an instance after it's launched. Each secondary VNIC can be in a subnet in the same VCN as the primary VNIC, or in a different subnet that is either in the same VCN or a different one. However, all the VNICs must be in the same availability domain as the instance."
https://docs.cloud.oracle.com/en-us/iaas/Content/Network/Tasks/managingVNICs.htm