Hi Ben
We had success in implementing something very similar. We created a custom Marketo form, which had many fields, including radio buttons. When the user clicked on Submit, another Marketo form opened in a pop-up on the same page with more fields.
This pop-up form had 2 different submit buttons and each button was associated with a different Thanks page.
So, it was a custom Marketo form, with another Marketo form in a pop-up on same landing page, and then 2 different thanks page.
Drop me a line at alok@grazitti.com and I can show you the demo for this.