The tricky part is going to be the controller logic and how to process a second taster if one exists. I would suggest getting that working first with a static form. You're going to need to insert a custom ield and then do a db insert if it is populated.
Then, it's just a matter of showing and hiding the field. Something like this: http://web2py.com/book/default/chapter/10#Conditional-Fields-in-Forms
