caetus,
если нужен пробел коментарий убрать
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
</head>
<body>
<script>
function kjhgs() {
var summa = [];
var go = document.getElementsByClassName('vse');
for (var i = 0; go[i]; i++) {
go[i].value && summa.push(go[i].value);
}
document.getElementById("syda").value = summa;
//document.getElementById("syda").value = summa.join(', ');
}
</script>
<input value="" id="syda">
<select class="vse" onchange="kjhgs();">
<option value=""></option>
<option value="один">1</option>
<option value="два">2</option>
<option value="три">3</option>
</select>
<select class="vse" onchange="kjhgs();">
<option value=""></option>
<option value="один">1</option>
<option value="два">2</option>
<option value="три">3</option>
</select>
</body>
</html>