One possibility outside of using a web service is to add two new Yes/No columns (YN1 and YN2) to the list/library both with a default value to No.  Then have the SPD workflow run and change the YN1 column to Yes.  Configure your Nintex workflow to run when an item is modified.  Next use a Run If action configured in the Nintex workflow to run its contents if YN1 = Yes and YN2 = No.  Last step in the Run If action would be to set YN2 to Yes (or both YN1 and YN2 back to No) so you don't get stuck in a loop.

This should not work I believe - Since SharePoint Designer and Nintex Workflows both use the SharePoint 2013 workflow engine, and SharePoint prevents workflow updates from triggering additional workflows.

The easiest option I believe would be to publish the Nintex workflow as a Component Workflow with Public API enabled, this will give you an HTTPS endpoint to post to at Nintex where we will start the workflow. If you need to send data in you can do that, though formatting the JSON payload may be a little difficult in SPD.

Callum

I was concnered that modifying the item would not allow the  2nd WF to kick-off but it does.  Thanks.

I will keep this trick in my backpocket.  I was concnered that modifying the item would not allow the  2nd WF to kick-off either but it does after some testing.