By default, when you send a message, the orchestration simply writes the message to the MessageBox and continues. The message is then processed independently by a send pipeline and adapter. By using another orchestration to perform each send you increasing the likelyhood of the message being sent out of order.
What you need to do is from a single orchestration send the messages using delivery notification on the port. This will force the orchestration send port to block (wait for an Ack/Nack) before continuing.