How many questions are on your second form? Just one asking them why they are unsubscribing?
If that's the case, I'd just have that field "why are you unsubscribing?" invisible on the form until someone selects "Unsubscribe" checkbox. Then that field can appear and they can fill it in before submitting. Here's a doc that shows how to 'hide' the field until someone checks the unsubscribe button: Dynamically Toggle Visibility of a Form Field - Marketo Docs - Product Docs
When you create a form, at the "Setting" stage of the process, you can have a conditional follow-up page:
Unfortunately, this is only available at form level, not at landing page level.
The other way to do this is to create a small JS, using the forms 2.0 API, that will change the follow-up based on the entered values. It takes some basic to average JS skills. The second example in the developers doc provides the basics for this : http://developers.marketo.com/rest-api/assets/forms/examples/