JSFiddle - React, Tailwind, and code Playground

HTML

<select name="cant_horas" class="form-control" onChange="multiplicar(); cambiaotro()"  id="txt1" onkeyup="sum();" required>
                                                                <option value="opt1">Seleccione</option>
                                                                <option value="1">1</option>
                                                                <option value="2">2</option>
                                                                <option value="3">3</option>
                                                                <option value="4">4</option>
                                                                <option value="5">5</option>
                                                                <option value="6">6</option>
                                                                <option value="7">7</option>
                                                                <option value="8">8</option>
                                                                <option value="9">9</option>
                                                                <option value="10">10</option>
                                                                <option value="11">11</option>
                                                                <option value="12">12</option>
                                                                <option value="13">13</option>
                                                                <option value="14">14</option>
                                                                <option value="15">15</option>
                                                                <option value="16">16</option>
                                                                <option value="17">17</option>
                                                                <option value="18">18</option>
                                                                <option...

JavaScript

function cambiaotro(){
var primero = document.getElementById('txt1').value;
var segundo = document.getElementById("copiar");
segundo.value = primero;
}

function multiplicar(){
alert("funcion multiplicar y pasa el mismo valor a select #2");
}