Currently, only one NS record per name is allowed. However, NS is unlike CNAME or PTR, multiple NS records are perfectly valid and is a common use case: multiple redundant DNS servers is a common configuration and should be supported. E.g.

set service dns forwarding authoritative-domain example.com records ns foo target 'ns1.example.com'
set service dns forwarding authoritative-domain example.com records ns foo target 'ns2.example.com'

should create two NS records and not overwrite the first one with the second.