Clustering the Device
I have not seen any procedure from RSA to make ES servers a part of cluster. But for HA, you can configured event sources to send events to both the ES servers which I have seen configured in my past experience.
As per my knowledge, cluster is supported only in EA configuration for Local Collectors. It may have been supprted in the upcoming versions i.e. 5.x
Having two ES's clustered with a virtual IP is not supported, if you are looking for high availability I'd recommend you to use a virtualized ES and then rely on the HA options that VMware offer.
Also you could check with RSA Professional Services to see if they can customize a solution for your environment.
I hope this helps