Mirror fields

HTML

Type something: 
<input id="a" type="text" class="mirror cba" formirror="abc"  placeholder="one">
<input id="b" type="text" class="mirror abc" formirror="cba" placeholder="two">
<input id="c" type="text" class="mirror bad" formirror="dab"  placeholder="one">
<input id="d" type="text" class="mirror dab" formirror="bad" placeholder="two">

JavaScript

$('.mirror').on('keyup', function() {
    $('.'+$(this).attr('formirror')).val($(this).val());
});