Luis,
As we know that there are multiple nodes in a RAC setup, all pointing to a single database. But the backup of database should be initiated from a single node, usually the primary node. The primary node is the one you setup first and do most of the maintenance from that node.
Youll need two UNIX client Licences - used to backup each nodes "flat" files and one Oracle for the Primary Node.
Are your servers configured in a SAN? If they are some SAN's have software to allow for BCV's and or Snap's, thi s is another thought.
JD