iframe更改网址取决于下拉值

我有2个iframe，分别为iframe 1和2。iframe1上还有一个下拉菜单，其中包含3个下拉菜单项。无论用户选择哪种方式，iframe都会通过将src动态替换为项目的值来显示网址。但是，对于第二个iframe，src取决于第一个iframe的src。第一iframe的src值将分别更改第二iframe的src。这是我无法使其工作的部分。 第一个iframe：

<!--script for the dropdown to different URLS-->
<div id="safety-declaration" class="page">
    <script type="text/javascript">
        function setIframeSource() {
            var theSelect = document.getElementById('location');
            var theIframe = document.getElementById('safety-declaration_form');
            var theUrl;
            theUrl = theSelect.options[theSelect.selectedIndex].value;
            theIframe.src = theUrl;
            console.log(document.getElementById('safety-declaration_form').src);
        }
    </script>

    <div id="safety-declaration_top">
        <!--dropdown-->
        <div class="dropdown">
            <select id="location" onchange="setIframeSource()">
                <option value="">--Please choose your unit--</option>
                <option value="https://youtube.com">A</option>
                <option value="https://facebook.com">B</option>
                <option value="https://stackoverflow.com">C</option>
            </select>
        </div>
    </div>
    <iframe id="safety-declaration_form" src=""></iframe>

第二个iframe：

<div id="safety2test">
            <script type="text/javascript">

                function urlchanger() {
                    var theIframe = document.getElementById('workout-declaration_form');
                    var unit = document.getElementById('safety-declaration_form').src;

                    if (unit == "https://facebook.com") {
                        theIframe.src = "https://www.dell.com/";
                    }
                    else if (unit == "https://youtube.com") {
                        theIframe.src = "https://www.americanas.com.br/";
                    }
                    else {
                        theIframe.src = "https://www.bbc.com/";
                    }
                }

            </script>
            <iframe id="workout-declaration_form" src=""></iframe>
        </div>