We do have the restriction that a single PayPal account would only work for one FormsCentral account. However, you are allowed to register the same PayPal account in another FC account, but the forms in your current FC account would lose their payment ability.
I tried the same scenario as yours: create a payment form, share it to another user, duplicate that form in another user account, and register the same PayPal account in that account. Then I can pay with the form in another user account. Everything works fine to me.
The problem you saw might be due to an known bug, which we have been fixed a while ago. Your document might have been created before the bug fix, so it still has the problem. Do you mind sharing the form to me (firstname.lastname@example.org) so that I can verify if the problem is due to the one I mentioned?
To get around the current problem so that you can continue your work, please do this:
1. In your old account, open the form you want and export the form design with File->Export Design File and save the .fcdt file in a temp folder.
2. Log in to your new account, and import the design file in Form Organizer with File->Import Design File.
3. Open the imported form and register the desired PayPal account.
Please let me know if this does not work for you.
Senior Computer Scientist
Adobe Systems Incorporated
The export / import works, thanks for that. I have shared the problematic form with you.