Compare two meta_values in WP_Query

functions.php

<?php 

add_filter( 'posts_request', 'gist_posts_request' );

function gist_posts_request( $sql ) {
    if ( strrpos( $sql, 'value_of_' ) === false ) {
        return $sql;
    }

    $matches = array();
    preg_match_all( "/'value_of_[^']*'/", $sql, $matches );

    foreach ( $matches as $match ) {
        $meta_key = substr( trim( $match[0], "'" ), 9 );

        preg_match( "/mt[0-9]*.meta_key = '$meta_key'/", $sql, $meta_key_match);

        if ( empty( $meta_key_match ) ) {
            continue;
        }

        $meta_key = explode( '=', $meta_key_match[0] );
        $meta_key = trim( $meta_key[0] );

        $sql_to_add = "CAST($meta_key AS SIGNED)";

        $sql = str_replace( $match[0], $sql_to_add, $sql );
    }

    return $sql;
}

/*
WP_Query 'meta_query' arg should be like this (with 'value_of_{meta_key}'):

'meta_query' => array(
    'relation' => 'AND',
    array(
        'key'           => 'meta_key_one',
        'value'         => 'value_of_' . 'meta_key_two',
        'compare'       => '>',
        'type'          => 'NUMERIC',
    ),
    array(
        'key'           => 'meta_key_two',
        'compare'       => 'EXISTS',
    )
),
*/