AJAX post submit actions

Hello,
I am currently writing a script which is supposed to take a user inputted registration form and if some of the information already exists in a database (MySQL) then copy it to a set of different fields on the form.  I am using AJAX to perform the action so the page doesn't have to reload but I am having some problems with it.  I'd also like to be able to return a generic error if something else goes wrong.  I think the code below will explain it better, I'm putting in comments of where I'm trying to make things happen.  My question is; what is the best way to have multiple actions happen?  I've gone through some tutorials but usually it's only a single operation which takes place when the AJAX request is performed.  Thanks in advance and let me know if you need any other information.
P.S. I am using Kohana as a framework, but I don't think that should matter for the actual AJAX programming.

public function action_register()
    {
        if( $this->request->is_ajax() )
        {
            $this->auto_render = FALSE;
        }
        if( $this->request->method() === Request::POST )
        {
          
            $user = new Model_User;
            $post = $this->request->post();


            if( $user->where( 'email', '=', $post[ 'Email' ] )->count_all() === 0 )
            {
                
                $userparams = array();
                $userparams[ 'first_name' ] = $post[ 'FirstName' ];
                $userparams[ 'last_name' ] = $post[ 'LastName' ];
                $userparams[ 'email' ] = $post[ 'Email' ];
                $userparams[ 'emailconf' ] = $post[ 'Emailconf' ]; 
                $userparams[ 'password' ] = $post[ 'Password' ];
                $userparams[ 'passconf' ] = $post[ 'Passconf' ];
                $usercheck = $user->validate_create( $userparams );

                try
                {
                    $usercheck->check();
                    $user->values( $userparams );             
                    $user->save();
                    $user->add( 'roles', ORM::factory( 'role' )->find(1) );
                    Auth::instance()->login( $this->request->post( 'Email' ), $this->request->post( 'Password' ) );
                    $this->redirect( 'userhome' );
                }
                catch( ORM_Validation_Exception $e ) // Registration validation error
                {   
                    // Return an error asking user to confirm correct values (possibly additional processing)
                }
            }
            else // Email exists
            {
                // Copy 'Email' value to 'LoginEmail' form element on main page.
            }

        }
        else
        {
            // Request was not a correct POST request
        }

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Script Code:

$(document).ready( function()
{
    $( '#register_form' ).submit( function( e )
    {
        e.preventDefault();
        var fName = document.getElementById( "FirstName" ).value;
        var lName = document.getElementById( "LastName" ).value;
        var Email = document.getElementById( "Email" ).value;
        var Emailconf = document.getElementById( "EmailConf" ).value;
        var Pass = document.getElementById( "Password" ).value;
        var Passconf = document.getElementById( "PassConf" ).value;
        var dataString = 'FirstName=' + fName + '& LastName=' + lName + '& Email=' + Email + '& Emailconf=' + Emailconf + '& Password=' + Pass + '& Passconf=' + Passconf;
        $.ajax(
                {
                    type: "POST",
                    url: "index.php/register",
                    data: dataString,
                    success: function( data )
                    {
                        alert( data );
                        // Separate Email Copy and recheck form
                            
                        }
                    }
                    
                });
    });
});

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