Is it possible to apply FAPI #states to profile2 fields on the user registration form?
If you want to alter a form, you need to create a custom module and implement hook_form_alter (or hook_form_FORM_ID_alter) in it. In your hook_form_alter implementation you will have the $form array available. Look up the element you're interested in and add a #states property to it.

The above is just theory; I have not tried it specifically with the profile2 form and a #states property, but I assume that it would work.