If you are using parallel shape then, Orchestration will not proceed unless both the branches complete the execution. Once execution of parallel branch is complete, then only it will proceed further and write something to send port. In your case I think you are expecting one message will be received at a time from either of receive port, then to make it work you have to use correlation.
In current situation, if you check HAT, you will see that your Orchestration instances are Dehydrated (as they were waiting for other receive port to receive something).