How to create 2 or more different registration pages in Drupal 7
Could Rules and Webform suffice for what you need to accomplish with the second registration form?

I actually did something similar recently, using a webform as a second registration form. I used the original registration form for the basic registration and on the second form I used Webform Rules to perform actions when the webform was submitted. Of course, this could also be done with a simple fielded node and Rules, but I liked the extra features that webform offered in doing this. If this project can be accomplished with nodes/Rules or Webform/Webform Rules then that can certainly offer you a lot of flexibility in updating the forms in the future as well.