How do I sort a custom post type admin column using two meta keys?

I have created a custom post type along with custom columns. One of my columns displays a date range such as “12/05/2012 – 11:00 am to 12/15/2012 – 1:00 pm”. This is a concatenated string using a combination of four fields: startDate, startTime, endDate, endTime.

So far I have been able to make this column sortable based upon the startDate meta_key, but I would like to extend this such that it’s sorted by the startDate and then by the startTime.

Currently

• 12/23/2012 – 7:00 pm
• 12/05/2012 – 11:00 am
• 12/05/2012 – 2:00 pm
• 11/30/2012 – 1:00 pm

Need

• 12/23/2012 – 7:00 pm
• 12/05/2012 – 2:00 pm
• 12/05/2012 – 11:00 am
• 11/30/2012 – 1:00 pm

This is my progress from what I’ve gathered over the internet so far, but it does not sort by time:

add_filter( 'request', 'event_column_orderby' );

function event_column_orderby ( $vars ) {

    if ( !isset( $vars['orderby'] ) || isset( $vars['orderby'] ) && 'datetime' == $vars['orderby'] ) {
        $vars = array_merge( $vars, array(
            'orderby'       => 'meta_value',
            'order'         => 'ASC',
            'meta_query'    => array(
                array(
                    'key'   => 'rqp-event-startDate'
                ),
                array(
                    'key'   => 'rqp-event-startTime'
                )
            )
        ));
    }

    return $vars;
}