Hi Peter,
I do not see any restriction in license for this configuration.
VPN load balancing requires an active 3DES/AES license. The security appliance checks for the existence of this crypto license before enabling load balancing. So you may have to activate 3DES/AES license. Other than that It does not look like you need any extra license.
You can refer the below link for this. Please refer for the cluster configuration.
http://www.cisco.com/c/en/us/td/docs/security/asa/asa82/configuration/guide/config/vpnsysop.html#wp1048834
P.S. Please rate helpful posts.
Thanks,
Shivapramod M