clusterconfig isn't an option in WSA.
You don't need a seperate key for clustering.
Clustering on the WSA is based on how you get the traffic to the box, they two boxes don't actually talk to one another.
If you use WCCP, you can be in active/active mode. I've not fiddled with it too much, but active/standby might be an option.
If you use a PAC file or direct proxy settings, you can be in active/standby or active/active, depending upon what other gear you have available... If you have a DNS load balancer, where the DNS server hands out addresses based on whether something is up or not, you could be active/active or active/standby... Or if you use a content switch, and put the WSA's behind it, you could get either active/actice or active/standby.
My opinion is that WCCP is your best option... Its cleaner...
