Código HTML:
<SCRIPT LANGUAJE="JavaScript">
function enviarForm1(form) {
form.action = "Pagina_1.htm";
form.target = "_blank";
form.submit();
}
function enviarForm2(form) {
form.action = "Pagina_2.htm";
form.target = "_blank";
form.submit();
}
function enviarFormNormal(form) {
form.action = "envioNormal.htm";
form.target = "_self";
form.submit();
}
</SCRIPT>
<FORM NAME=formulario action="" target="_self">
<input type=text name=nombre value="Introduce tu nombre" SIZE="20">
<input type=button value="Enviar Datos Normal" onClick="enviarFormNormal(this.form)">
<input type=button value="Enviar Datos a Dos paginas" onClick="enviarForm1(this.form); enviarForm2(this.form);">
</FORM>
Espero que te sea útil!