Sounds like you found the right plugin!
Here’s an overview of what steps you can take;

1. Using Formidable Form’s plugin – create one form with fields for all the required information to complete both tax forms. (Note: You must submit at least dataset record to use the PRO2PDF plugin)
2. Using http://www.PDFEscape.com – add PDF Form Fields to blank W-9 and W-2 forms.
3. Using Formidable PRO2PDF  – upload the PDF Forms and create two Field Maps – one for each PDF Form.
4. Place both PRO2PDF shortcodes in the Formidable Form’s On-Submit message, but change the dataset=’xx’ to dataset=[id] so the user can download, print or email their forms.

Let us know which steps you need more detailed instructions.