The ApplicationRegistry can currently have multiple instances, in order to cater for multiple brokers in the same VM. However, many parts of the code simply call ApplcationRegistry.getInstance() to obtain a reference, meaning multiple instances are ignored. The only time this is ever useful is for certain types of tests, therefore the capability to have multiple broker instances should be removed.
Any tests that require multiple brokers can start a VM broker and an external, or two external brokers. In general, test-only code should not be made available to the rest of the application.