In the case of multiple subnets, it is not required to have as many NFS filesystems/volumes as the subnets for dNFS to be effective. Also, it is not required to mount a single volume on to all subnets for dNFS to be effective either. Oracle dNFS reads the oranfstab and based on the storage paths and mount details, it will create multiple paths when the database files are accessed.
For example, with two subnets and two mounts, dNFS would create four paths to the storage system for every server process.