Custom redirect based on form fields?

I am looking for some specific custom behavior from a WP form, and want to know a) if this is possible, and b) if there is an existing plugin that does it (or can be easily modified to do so):

Basically, I want a form on the site which will display a custom message depending on whether or not certain form elements were filled out or not.

Specifically, there would be two checkboxes in the form – if BOTH are checked then the user would get a very specific ‘thank you’ message when the form is submitted. If only one, or neither of the boxes are checked, they get a different (default) thank-you.

Any and all help would be appreciated.