Sounds like you found the right plugin!
Here’s an overview of what steps you can take;
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)
Using Formidable PRO2PDF – upload the PDF Forms and create two Field Maps – one for each PDF Form.
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.