Do I need to have two queues for Service Broker?
You do not have to, but is a good practice.

You must be aware that your 'from' service will also get messages in it's queue, even if you never send any. There are system messages, most obvious ones being the EndDialog and Error messages. You have to handle at least those. So your code that services the queue must be able to properly handle the messages sent to the initiator (the 'from' service). With a single queue for both services you will get a mix of messages, some for initiator some for target.


  will have code in a trigger to send a simple empty message


Make sure you do not do Fire and Forget, read How to prevent conversation endpoint leaks to understand why is important.