User Query Multiple Orderby Clause

Try using:

'orderby'=>'distribution_list last_name',
'order'=>'ASC'

Instead of:

'orderby' => array(
    'distribution_list' => 'ASC',
    'last_name' => 'ASC',
),

So:

$users = new WP_User_Query( array(
    'meta_query' => array(
        'relation' => 'AND',
        'lastname' => array(
            'key' => 'last_name',
            'compare' => 'EXISTS',
            'type' => 'CHAR',
        ),
        'list' => array(
            'distribution_list' => array(
                'key' => 'distribution_list',
                'value' => '"72"',
                'compare' => 'LIKE'
            ),
            'distribution_list2' => array(
                'key' => 'distribution_list',
                'value' => '"72"',
                'compare' => 'NOT LIKE'
            ),
            'relation' => 'OR',
        ),
    ),
    'orderby'=>'distribution_list last_name',
    'order'=>'ASC'
) );