http://boo.2ch.net/ を見ると

function Cookies_get(){
        cookies = document.cookie;
        cookies = cookies.replace(/^.+boo2008_trip=/,'');
        cookies = decodeURIComponent(cookies);
        document.boo2008_red.boo2008_trip.value = cookies;
        document.boo2008_blue.boo2008_trip.value = cookies;
}

ってなってるんで，順番的に boo2008_trip の後ろに来る Cookie は全部入っちゃいますね．
↓な感じ？

function Cookies_get(){
        var value = /(?:^|[,;]\s*)boo2008_trip=([^,;\s]+)/.exec(document.cookie);
        document.boo2008_red.boo2008_trip.value
            = document.boo2008_blue.boo2008_trip.value
            = value ? decodeURIComponent(value[1]) : "";
}