JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" id="input1" onkeyup="change(2)" />
<input type="text" id="input2" onkeyup="change(1)"/>

JavaScript

change = function (input) {
    elem = document.getElementById("input" + input);
    if (input == 1)
        elem.value = document.getElementById("input2").value
    else
        elem.value = document.getElementById("input1").value
}