1. For Scaling Purposes, YES, SOA 11g can have any number of server in a Cluster with a load balancer in front of this. Clustering is pretty much done using the same weblogic cluster concepts even for SOA Env also. Using config wizard, create a SOA/BPM Domain. At one point, you can add "n" number of soa servers and put these in a Cluster. Cluster is just logical grouping of all soa servers. Now each soa server can be on the same physical box, or totally different remote physical boxes. Once you have a Clustered BPM Domain, you can put software (apache http server that goes very well with weblogic server) or any hard ware load balancer in front of this cluster. All the user will now access this load balancer url (host:port/someurl). And Load balancer will redirect the request to back end clustered soa servers in many ways like round robin etc. This provides high scalability and also Fail over like one machine dies, still other machines will be working. requests will not be send to dead machiine etc.
You can scale a single engine cluster as Ravi described but you no longer have the concept of multiple BPMN engines. You get one BPMN engine per SOA/BPM Weblogic Domain in 11g, so if you want a separate engine you have to have a 2nd domain along with its own set of MDS and SOA-INFRA schemas. MDS is mostly the equivalent of FDI in 10g.