Questo è un problema di javascript,

Prova cosi',

<script>
function OnButton1()
{
document.form1.action = "pagina2.php"
document.form1.target = "_blank";
document.form1.submit();


document.form1.action = "pagina1.php"
document.form1.target = "_self";
document.form1.submit();

return true
}
</script>



<form name="form1" method="post">

<input type=text name="campo1" value="...">
<input type=text name="campo1" value="...">
<input type=text name="campo1" value="...">

<input type="submit" onclick="OnButton1();" >
</form>