I wouldn't use an iframe for this, that will cause issues. instead us Framework builder to add the registration_prof.php and registration_student.php to the registration page.
The idea of framework builder plugins to create separate pages with their own functionality, then add them to another page.
Get the registration_prof and student pages working separately, then add them to the registration page using plugins.
where you are going to have 2 form on the same page, it is important that each form use unique form element names, especially the submit button, and that each server behavior that will process these forms use the submit button as the trigger.
looking at your pages, i see a few issues:
1) the paths to support files on the registration_prof page looks wrong:
<link href="../../../Library/WebServer/Documents/ICMIDS/Styles/SiteStyle_incl.css" rel="stylesheet" type="text/css" />
on the student page, that same file is included as:
<link href="Styles/SiteStyle_incl.css" rel="stylesheet" type="text/css" />
this is also effecting the forms action. make sure both of these files are being created in the same location.
2) the insert behavior on registration_prof does not have a name for the session to save the inserted ID:
this is needed to return the recordset on the next page
3) Student registration uses the any form post trigger for the insert, it should use the Submit button pressed trigger
4) register_psucheckout has an insert and validation behavior on it, these are not needed as nothing will be submitting to this page.