Classic WebSphere supports HTTP filters that support setting the Kerberos Realm Name, so that SPNs from different realms can be used:
krb5Realm: This parameter is not required. Use to supply a Kerberos realm name. If the krb5Realm parameter is not specified, the default Kerberos realm name in the Kerberos configuration file is used.
So, a single WAS server can support multiple REALMs.
Is this possible in Liberty? Could this be handled by specifying mutliple spnego elements?
The multiple spnego elements is suggested in the following link which sets an id for the spnego element, which suggests multiple can be set:
Is setting the Realm Name in the servicePrincipalNames supported?