Rasy,
<body>
<meta charset="utf-8">
<div class="new-grav">
<button class="btn-grav on" id="activevvod-1">
Гравировка на футляре
</button>
<input id="grav1" placeholder="Гравировка на футляре" disabled>
</div>
<div class="new-grav">
<button class="btn-grav" id="activevvod-2">
Гравировка на кошельке*</button>
<input id="grav2" placeholder="Александр" disabled>
</div>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.2.4/jquery.min.js"></script>
<script>
$('.new-grav').each(function() {
var context = this,
$btn = $('button', context),
$inp = $('input', context);
$btn.on('click', function () {
$inp.prop('disabled', !$inp.prop('disabled'));
});
});
</script>
</body>