You could use javascript to do this.
<HTML>
<HEAD>
<TITLE>test</TITLE>
<SCRIPT LANGUAGE="JavaScript">
<!--
function open_windows()
{
url1 = "http://"+document.myform.username.value+":"+document.myform.password.value+"@[url]www.mysite1.com/index.html[/url]";
url2 = "http://"+document.myform.username.value+":"+document.myform.password.value+"@[url]www.mysite2.com/index.html[/url]";
window.open(url1, "window1");
window.open(url2, "window2");
}
-->
</SCRIPT>
</HEAD>
<BODY>
<FORM NAME='myform'>
<INPUT TYPE='text' NAME='username'><P>
<INPUT TYPE='text' NAME='password'><P>
<INPUT TYPE=button VALUE="click" onclick="open_windows()">
</FORM>
</BODY>
</HTML>
This assumes you are using the same username and password for each login.