Edit in JSFiddle

$(document).ready(function() {
    var viewModel = function() {
        var cel = ko.observable();
        var fah = ko.computed({
            read: function() {
                return (cel() * 9 / 5) + 32;
            },
            write: function(value) {
                cel(((value - 32) / 9) * 5);
            },
            owner: this
        });

        return {
            cel: cel,
            fah: fah
        };
    }();

    ko.applyBindings(viewModel);
});

<span>Temperature in Celcius : </span><input data-bind="value:cel" />
<br /><br/>
<span>Temperature in Fahrenheit : </span><input data-bind="value:fah" />