As long one JVM can handle the load you might use a.
But you might run into issues that the JVM get stuck by long GC periodically or did not have enough memory (32Bit Java?). In this case you can spit in two instances and build a cluster.
In my experience two instances on the same machine will work if you have enough memory and CPU power.
But you need to verify whether the system can handle the load with the resources.