A voting disk is a critical file for Oracle Clusterware. You must have at least 3 voting disks. If you have „only“ two SAN then you can configure the third on a NFS share.
Oracle supports now a third voting disk on NFS for HPUX. But only for HPUX 11.31 and Oracle Clusterware 10.2.0.2 onwards.
This white paper describse how to configure it:
Using standard NFS to support a third voting disk on an Extended Distance cluster configuration on Linux,AIX,HP, or Solaris (PDF)