With a simple, sequential workflow, correlated on an argument to the method (ie content-based correlation), with OperationA (CanCreateInstance = true, CorrelatesOn=argument) and OperationB (CanCreateInstance = false, CorrelatesOn=argument), a second client call to OperationA causes "operation is not available at this time" (as expected).
When OperationB is moved into a more complex activity (eg Pick or StateMachine), a second call to OperationA causes a TimeoutException.
When the correlation settings are moved to the CorrelationInitializer (which I suspect is the recommended approach), the workflow fails even more spectacularly, rendering the correlated workflow unusable and requiring the instance to be manually removed from the persistence store.
A comprehensive sample project is attached. I'd be glad to discuss it further with someone on the WF team.