JSFiddle - React, Tailwind, and code Playground

HTML

<select id="firstmenu" name="City">
<option value="--">--</option>
<option value="NY">NY</option>
<option value="LA">LA</option>
</select>
<br/>

<input type="text"  name="zipCode" id="secondmenu">
<select id="secondmenu" name="zipCode">
<option value="10001">10001(NY)</option>
<option value="90001">90001(LA)</option>
</select>

JavaScript

window.onload = function() {
	
	document.getElementById("firstmenu").addEventListener('change',function(e) {ha(e,'secondmenu');}, false);
	document.getElementById("secondmenu").addEventListener('change',function(e) {ha(e,'firstmenu');}, false);
};
function ha (e,id){
	var index = e.target.selectedIndex;
	document.getElementById(id).selectedIndex = index;
}