It's a generally good idea to have multiple balancers as you have a single point of failure if you dont do so. With mod_jk where the server doesnt even know about the balancer its enough to setup the balancers and voi-la, but it cant be any more difficult with mod_cluster!
In mod_cluster most things are autoconfigured if you are using multicast for the advertisements. The difference with having 2 balancers is that the AS will have to send updates to both servers so it has to have the knowledge of both. So, if you are using mutlicast just make sure you start the same configuration on the 2nd balancer and that the balancer is also receiving multicast messages and everything will be configured automatically -- no extra work required here.
If you configure it yourself for some reason, there is attribute proxyList. It is comma deliminated list of balancers to register and send updates to. See the docs for more info if you should need it http://docs.jboss.org/mod_cluster/1.0.0/html/javaproperties.html
HTH also for better understanding of mod_cluster,
We're going to use Symantec Veritas Cluster Suite (VCS) for the Apache servers and just fail from one to the other in the event of one dying.
Similar fencing/failover servers from other vendors should also work (e.g. RHEL High Availability Add-on includes what used to be called Red Hat Cluster Suite).
I'm using the Zen LoadBalancer appliance (can be physical or virtual machine) as the service load balancer to the apache/mod_cluster.
In this way I have HA and Load balancing.