This isn't really server-side, you're just registering some JavaScript to run on page load. So just update the JavaScript to open two windows:
string url1 = "Billview.aspx", url2 = "Whatever.aspx?somestuff=42";
string js = "window.open('" + url1 + "', '_blank', 'height=350,width=475,status=yes,toolbar=no,menubar=no,location=no,scrollbars=yes,resizable=no,titlebar=no' );\n" +
"window.open('" + url2 + "', '_blank', 'height=350,width=475,status=yes,toolbar=no,menubar=no,location=no,scrollbars=yes,resizable=no,titlebar=no' );
ScriptManager.RegisterStartupScript(this, typeof(string), "OPEN_WINDOW", js, true);