Separate User Groups / forms
Why do you want to let them register twice? Maybe re-think the way you go about this:

Possible solutions:


Add them to a certain user group (via a plugin) on registration with the appropriate permissions and/or...
...if you don't want to write a plugin, you can add a certain category to the user element on registration. You can than check throughout your site if the user has the correct category ('permission') to do something. If they come back and want to become a full member, just let them fill out a new form which UPDATES their already existing user account with a new category/permission.


That's the way I used it on a large site and it works rather fine.