così?

codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>


<script>
var campo1 = true;
var campo2 = true;


function check_campo1() {
    var c_numero = document.getElementById('numero');

    if (isNaN(c_numero.value) || (c_numero.value.replace(/\s/g, '') == "")) {
        c_numero.style.background = "#cc0000";
        c_numero.focus();    
        campo1 = false;
        return false;
    }
    
    campo1 = true;
    c_numero.style.background = "#fff";
}

function check_campo2() {
    var c_testo = document.getElementById('testo');

    if (c_testo.value.replace(/\s/g, '') == "") {
        c_testo.style.background = "#cc0000";
        c_testo.focus();  
        campo2 = false;
        return false;
    }
    
    campo2 = true;
    c_testo.style.background = "#fff"; 
}


function check() {
    return (campo1 && campo2);
}
</script>



<title>Documento senza titolo</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
</head>

<body>

<form action="" name="modulo" onSubmit="return check();">
<input type="text" name="numero" id="numero" onblur="check_campo1()">inserire un intero

<input type="text" name="ok" id="testo" onblur="check_campo2()">campo obbligatorio 
<input type="submit" value="OK"> 
</form>
</body>
</html>