There is an incorrect description in the following documentation:

https://docs.redhat.com/en/documentation/openshift_container_platform/4.20/html/multiple_networks/primary-networks#about-udn_about-user-defined-networks

--> You can assign one or more UDNs to a namespace, with a limitation of only one primary UDN to a namespace, and one or more namespaces to a UDN.

Since UserDefinedNetwork CR is namespace scoped and you can not assign multiple namespaces to a UDN.