function showType(str){
    if (window.XMLHttpRequest) {// code for IE7+, Firefox, Chrome, Opera, Safari
        xmlhttp = new XMLHttpRequest();
    } else {// code for IE6, IE5
        xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
    }
    xmlhttp.onreadystatechange = function() {
        if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
            document.getElementById("coachType").innerHTML = xmlhttp.responseText;
        }
    }
    xmlhttp.open("POST", "findType.c?s="+str, true);
    xmlhttp.send();
    showType2(str);
}
function showType2(str){
    if (window.XMLHttpRequest) {// code for IE7+, Firefox, Chrome, Opera, Safari
        xmlhttp = new XMLHttpRequest();
    } else {// code for IE6, IE5
        xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
    }
    xmlhttp.onreadystatechange = function() {
        if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
            document.getElementById("subjectId").innerHTML = xmlhttp.responseText;
        }
    }
    xmlhttp.open("POST", "findType2.c?s="+str, true);
    xmlhttp.send();
}

<select id="coachId" name="coachId" onchange="showType(this.value),showType2(this.value)"></select>
<td><div id="coachType"></div></td>
<td><select id="subjectId" name="subjectId"><option>请选择教练</option></select></td>