It can be done without using parallel server, all you have to do is modify your tnsnames.ora file to look something like this...

net_service_name=
(description=
(failover=on)
(address=(protocol=tcp)(host=sales1-svr)(port=1521))
(address=(protocol=tcp)(host=sales2-svr)(port=1521)))
(connect_data=(service_name=sales.us.acme.com)))


hth.