>Can I have 2 servers with esb's and the same queue defined on them >and one single access point ?
Yes, if you use JBoss Messaging you can have distributed queues, which means that posting to a "single" queue will distribute the message over the queues in the cluster, and to ESBs that are part of the Messaging cluster.
> What does it mean clustering JBossESB over JMS?
Clustering JBossESB over JMS means that by using JBossMessaging the ESB can use a JMS provider that is serverless (does not rely on a central server). What I mean by this is that instead of having a single JMS Provider, which all running instance's of the ESB use, there can be a cluster of them. As the ESB acts a both a producer and consumer of messages this means that you get load balancing and failover for these producers and consumers that use JMS as their transport.
Kurt has been working on loadbalancing and failover for services withing the ESB will probably be able to answer your questions about that.
This information is also within the Administrator's Guide, available via the trunk. If that doesn't help, let us know so we can improve it.