title: How to map one attribute to two properties in litElement?
body: <p>I am working in a litElement project, and in a component, I have an attribute that needs to be mapped to a property, and computed with a function to another one, something like this:</p>
  <pre class="lang-js prettyprint-override"><code>
  const calculateTwo(val) {
   return `${val} ${val}`
  }
  
  class MyElement extends LitElement {
    
    static get properties() {
      return {
        one: {
          type: String,
          attribute: 'foo',
        },
        two: {
          type: String,
          attribute: 'foo',
          reflect: false,
          converter: value =&gt; calculateTwo(value),
        },
      };
    }
  }
  </code></pre>
  <pre class="lang-html prettyprint-override"><code>&lt;my-component foo=&quot;bar&quot;&gt;&lt;/my-component&gt;
  </code></pre>
  <p>If I do this, <code>one</code> is not setted with 'bar', but <code>two</code> is correct</p>
  <p>If I remove the property <code>two</code>, <code>one</code> works properly.</p>
  <p>What would be the better approach to achieve this?</p>
  <p>I can go with the <code>update</code> function, but I would like to know if there is a better approach.</p>
  <p>I don't want to use a getter function to one of the properties, because the function of the converter is pretty heavy and I don't want it to be called everytime that I want to access the prop.</p>