From a Session EJB method, you can sequentially place two messages on two message ques.
The two Message-Driven EJB that are connected to these message ques will process the message on their message que in their own thread.
Whatever logic that happens from each Message-Driven EJB will occur concurrently.
Keep in mind that
you should not code business logic in the Message-Driven EJB code. So, whatever logic that occurs with message should either be in a Session EJB or a POJO business object.