JSFiddle - React, Tailwind, and code Playground

HTML

<select id="productchoice" onchange="productchoicechange()">
   <option value="1">Option #1</option>
   <option value="2">Option #2</option>
  </select>

  <select id="productchoice2">
   <option value="1">Option #1</option>
   <option value="2">Option #2</option>
  </select>

JavaScript

function productchoicechange(){

    var productchoice = document.getElementById("productchoice");
    var productchoice2 = document.getElementById("productchoice2");
    productchoice2.options[productchoice.options.selectedIndex].selected = true;

}