I guess you could try to create an entry in EMM making the DAG virtual name an app cluster in NBU. And then tie the two physical nodes to that name. In the Exchange policy you should use the DAG name, and in theory, an Enterprise client license should work, unless changed in 7.x.
There is however some confusion regarding what is supported or not, but it is my understanding that clustering media server| SAN media server roles in EMM is not supported, but using the app_cluster is still supported for application backup.
If this does not work, then I guess you will need full NetBackup Enterprise Server license. Alternatively, as you have the Enterprise client license, you could opt for SAN client. This way the Exch nodes sends data over the SAN to one of your media servers, which then move to disk/tape. SAN client option does however impose some requirements on the media server side (using FT server feature), as only a select few HBAs are supported for target (see compatibility list).
Use following commands to associate your DAG, replace with relevant names;
nbemmcmd -addhost -machinename <name_of_virtual_dag> -machinetype app_cluster
nbemmcmd -updatehost -add_server_to_app_cluster -machinename <node1_in_Exch_cluster> -machinetype media -clustername <name_of_virtual_dag> -netbackupversion 7.1 -masterserver <master_name>
nbemmcmd -updatehost -add_server_to_app_cluster -machinename <node2_in_Exch_cluster> -machinetype media -clustername <name_of_virtual_dag> -netbackupversion 7.1 -masterserver <master_name>
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