@Tracked fullName

controllers.application\.js

import Controller from '@ember/controller';
import { action } from '@ember/object';
import { tracked } from '@glimmer/tracking';
import { cached } from 'tracked-toolbox';

class Person {  
  @tracked firstName = 'Jen';
  @tracked lastName = 'Weber';

  @cached
  get fullName() {
    return `${this.firstName} ${this.lastName}`;
  }
}

export default class ApplicationController extends Controller {
  user = new Person();
  
  @action
  updateFirstName(event) {
    this.user.firstName = event.target.value;
  }
  
  @action
  updateLastName(event) {
    this.user.lastName = event.target.value;
  }
}

templates.application\.hbs

<p>
  First name:
  
  <input
    value={{this.user.firstName}}
    {{on 'input' this.updateFirstName}}
  >
</p>

<p>
  Last name:
  
  <input
    value={{this.user.lastName}}
    {{on 'input' this.updateLastName}}
  >
</p>

<p>
  Full name:
  
  {{this.user.fullName}}
</p>

twiddle\.json

{
  "version": "0.17.1",
  "EmberENV": {
    "FEATURES": {},
    "_TEMPLATE_ONLY_GLIMMER_COMPONENTS": false,
    "_APPLICATION_TEMPLATE_WRAPPER": true,
    "_JQUERY_INTEGRATION": true
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.5.1/jquery.js",
    "ember": "3.18.1",
    "ember-template-compiler": "3.18.1",
    "ember-testing": "3.18.1"
  },
  "addons": {
    "@glimmer/component": "1.0.0",
    "tracked-toolbox": "1.2.3"
  }
}