Okay, let me reframe this a little if I may?
The initial workflow is one where you take a screenshot, and then have to, on occasion, change the format of the image so that you can use it with a particular web service. You have an Automator-based way of converting the file format, but you would like to streamline it further so you don’t have to manually trigger the format change. You also wan to retain your default image format.
Your proposed, ideal workflow, would always produce both image formats; but this would potentially leave you with few duplicate images over time, or at least extra deletions to carry out on a regular basis.
What if we took these as two different workflows? One where you want to take a typical screenshot, and one where you want to take one in a different format? That way you choose a different trigger for each different process.
You already have something built in Automator to convert a PNG to a JPG. Did you know Automator also has a ‘take screenshot’ action step? Did you also know that you can assign keyboard shortcuts (like the ones you use for screenshots) to Automator actions?
With this in mind, you might find these two blog posts of interest.