Auto applies Coupon w/ use of a different coupon.

coupon_multi.php

<?php 
/*
 * Auto applies coupon code based on another coupon code and removes all if either is removed
 */
function auto_apply_coupon() {

    $applied_coupon_code = 'Coupon manually applied goes here';
    $coupon_code = 'coupon you want auto applied';

    if ( WC()->cart->has_discount($applied_coupon_code) ) {

        WC()->cart->add_discount( $coupon_code );
        WC()->cart->calculate_totals();

    } elseif ( !WC()->cart->has_discount($applied_coupon_code) && WC()->cart->has_discount($coupon_code) ) {

        WC()->cart->remove_coupons();
        WC()->cart->calculate_totals();

    }
};
add_action( 'woocommerce_before_cart_table','auto_apply_coupon' );