To create a combined Login and Register page, simply follow these steps below. The following procedure may or may not work depending on your version of DotNetNuke. 

1. Login as Host and go to Admin > Site Settings and then under Advanced Settings followed by Host Settings, include the "Users and Roles" and "Account Login" modules. These modules will now be available to be added to a page just like other modules. 
2. Create a new page called "Login" and hide it from the menu. Make the page viewable by Administrators (default) and Unauthenticated Users.
3. Add the "Account Login" module and the "Users and Roles" module to the page. The DNN Users and Roles module comes with a bunch of module controls and you can remove the ones you don't need and keep only the one that looks like a registration form. Arrange them so they look nice.
4. Go to Admin > Site Settings and then under Advanced Settings and Page Management and set the Login selection to your newly created login page.