Description of problem:

As of now, it is required to create new bridge mappings with a unique name for every VLANs. So we cannot have two different NAD configuration pointing to same bridge maping. Please also refer  OCPBUGS-31679. 

Since, this is not explicitly prevented and its' not easy to find in the logs, we need to document this limitation at  https://docs.openshift.com/container-platform/4.15/virt/vm_networking/virt-connecting-vm-to-ovn-secondary-network.html#virt-creating-localnet-nad-cli_virt-connecting-vm-to-ovn-secondary-network