You should be able to invoke the workflow by adding a "Process Robot" to the ECA workflow and specifying the RMT Workflow as the target process. When this robot is executed it will start the RTM workflow. You should be careful here because if the ECA workflow is terminated it is possible that sub-processes spawned from the ECA will also be terminated.
Is it would intent to release to the downstream systems via ESI prior to completing the ECN? The alternative is to adjust the ECN life cycle such that it automatically initiates the RTM workflow once the ECN reaches a certain state. I suppose you could do the same for the ECA workflow but I would think you'd want all tasks completed and your data stable prior to transmitting it via the ESI process.