<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
<script src="https://cdn.rawgit.com/finom/balalaika/master/balalaika.min.js"></script>
<script>
$(function() {
$.index=function(elem){var i=0;while(elem=elem.previousSibling){elem.nodeType==1&&i++}return i}
var data = {},
output;
$('.child').on('keyup', function() {
data['input' + $.index(this)] = this.value;
output = Object.keys(data).map(function(e) {
return data[e];
});
main.value = output.join(' ');
});
});
</script>
</head>
<body>
<input type="text" value="" id="main" />
<input type="text" value="" class="child" />
<input type="text" value="" class="child" />
<input type="text" value="" class="child" />
</body>
</html>
вместо разделителя +, поставил пробел.