Default.aspx
<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
    <title>无标题页</title>    
</head>
<body>
    <form id="form1" runat="server">
    <div>
        <iframe src="HTMLPage1.htm" width="500px"                         height="200px"></iframe>
        <iframe src="HTMLPage2.htm" width="500px" height="500px"></iframe>
    </div>
    </form>
</body>
</html>
===============================================
HTMLPage1.htm
<html xmlns="http://www.w3.org/1999/xhtml" >
<head>
    <title>无标题页</title>
    <script type="text/javascript">
        function frameSetNum()
        {
            var text=window.parent.frames[1].document.getElementById("Text1");
            text.value=parseInt(text.value,10)+1;
        }
    </script>
</head>
<body>
    <input id="Button1" type="button" value="修改文本值" onclick="frameSetNum()" />
</body>
</html>
===============================================
HTMLPage2.htm
<html xmlns="http://www.w3.org/1999/xhtml" >
<head>
    <title>无标题页</title>
</head>
<body>
    <input id="Text1" type="text" value="0" />
</body>
</html>
===============================================
效果为:每点击一次“修改文本值”按钮。文本框里的数值加一。
不知道是不是你要的。