Stop flex items stacking on top of each other

Question

I'm trying to get to grips with flex and am struggling to create what I'm after.

What I'm trying to do

• Full screen container
• Two div elements, one right-aligned with a width of 640px and one left-aligned taking the remaining space

What's happening

My elements are displaying, on top of one another, in the centre of the screen.

Code

div.flex {
  display: flex;
  flex-direction: row;
  flex-wrap: nowrap;
  justify-content: space-around;
  align-items: stretch;
  height: 100vh;
}
div.hero {
  background-size: cover;
  background-position: center bottom;
  position: relative;
  height: 100vh;
  width: 100%;
  margin: auto;
}
div.timeline {
  width: 640px;
  margin: auto;
}
div.header {
  position: absolute;
  top: 50%;
  text-align: center;
  width: 100%;
  /* color: #fff; */
  -ms-transform: translate(0, -50%);
  /* IE 9 */
  -webkit-transform: translate(0, -50%);
  /* Safari */
  transform: translate(0, -50%);
  -ms-transform: translate(0, calc(-50% - 66px));
  /* IE 9 */
  -webkit-transform: translate(0, calc(-50% - 66px));
  /* Safari */
  transform: translate(0, calc(-50% - 66px));
}

<div class="flex">

  <div class="hero">

    <!-- Header -->
    <div class="header">
      <h1>Title</h1>
      <h2 class="subtitle">Subtitle</h2>
    </div>
    <!-- End header -->

    <!-- Timeline -->
    <div class="timeline">
      <ul class="timeline-both-side">
        <li>
          <div class="border-line"></div>
          <div class="timeline-description">
            <p>Quisque ac laoreet purus, eu dapibus ligula. Mauris nec tincidunt mi, eget finibus sem. Morbi viverra malesuada lobortis. Suspendisse sed augue luctus ex molestie convallis.</p>
          </div>
        </li>
        <li class="opposite-side">
          <div class="border-line"></div>
          <div class="timeline-description">
            <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit. Sed sed erat consectetur, tempor odio sit amet, euismod sapien.</p>
          </div>
        </li>
        <li>
          <div class="border-line"></div>
          <div class="timeline-description">
            <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit. Sed sed erat consectetur, tempor odio sit amet, euismod sapien.</p>
          </div>
        </li>
        <li class="opposite-side">
          <div class="border-line"></div>
          <div class="timeline-description">
            <p>Quisque ac laoreet purus, eu dapibus ligula. Mauris nec tincidunt mi, eget finibus sem. Morbi viverra malesuada lobortis. Suspendisse sed augue luctus ex molestie convallis.</p>
          </div>
        </li>
        <li>
          <div class="border-line"></div>
          <div class="timeline-description">
            <p>Quisque ac laoreet purus, eu dapibus ligula. Mauris nec tincidunt mi, eget finibus sem. Morbi viverra malesuada lobortis. Suspendisse sed augue luctus ex molestie convallis.</p>
          </div>
        </li>
      </ul>
    </div>
    <!-- End timeline -->

  </div>

</div>

Question

How can I use flex to have these two elements, both with 100vh, in a row like below?

+------------------------------------------+
|.flex                                     |
|+-------------------------+ +------------+|
||.hero                    | |.timeline   ||
||                         | |            ||
||                         | |            ||
|+-------------------------+ +------------+|
+------------------------------------------+

Answer 1

Simplicity itself.

body {
  margin: 0;
}
.parent {
  height: 100vh;
  display: flex;
}
.hero {
  flex: 1;
  background: red;
}
.timeline {
  flex: 0 0 640px;
  background: green;
}

<div class="parent">
  <div class="hero"></div>
  <div class="timeline"></div>
</div>

Codepen Demo