On your second PDF, you need to add the Process Management\Form Augmenter\Inject Form Bridge operation.
This will enable the communication from the PDF to Workspace.
I do send the second PDF through the Inject Form Bridge operation. My output from the renderPDFForm operation is stored in a document variable, then I use that document variable as the input and output of the Inject From Bridge operation. I have two exit routes from the assign task, and I do get two buttons at bottom of the workspace screen, one for each route. I even see the following added to my XML data:
Is there some configuration on the server that I've missed?