You can only have one named instance per virtual name. As of SQL 2008R2 you couldn't add a second instance to an existing virtual name.
Keep in mind the various names and ip addresses involved in clustering
Node 1 Name, Node 1 IP Address
Node 2 Name, Node 2 IP Address
Cluster Virtual Name, Cluster IP Address
SQL Virtual Name 1, SQL IP Address 1 (named [only one per cluster] or default instance)
SQL Virtual Name 2, SQL IP Address 2 (named [only one per cluster] or default instance)
It seemed from a previous post that the SQL was being referenced this way [cluster virtual name]\[Instance Name], it *may work as long as the instance is on the same node as the cluster virtual name is residing on. The cluster virtual name is basically just the administration name for the cluster in the case of SQL.