Why not put A2 in a different server group? Just make sure that there are no port conflicts you intend to run A2 on the same host. The number of hosts is an orthogonal concern to clustering. So long as your deployment names are the same on A and A2, you'll be able to balance your load to both servers.
The domain concept is different from farming, this was to distribute applications in a cluster in former JBoss versions.
A loadbalancer will do the job if it will prefere A if available. But remember if you use EJB invocation direct from a remote client the client might keep the reference to A2 and will not return.
You need extra configuration here.