You can use JavaScript window.parent object to access iframe parent page. Below is sample for you reference.
Page1.aspx
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
</head>
<body>
<form id="form1" runat="server">
<div>
<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
<iframe id="frame1" src='Page2.aspx' />
</div>
</form>
</body>
</html>
Page2.aspx
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
</head>
<body>
<form id="form1" runat="server">
<div>
<asp:TextBox ID="TextBox1" runat="server" onkeyup="SetParentValue()"></asp:TextBox>
</div>
</form>
<script>
function SetParentValue() {
parent.document.getElementById("TextBox1").value = document.getElementById("TextBox1").value;
}
</script>
</body>
</html>