<ul class="flex-container">
  <li class="flex-item flex1">1</li>
  <li class="flex-item flex2">2</li>
</ul>

.flex-container {
  padding: 0;
  margin: 0;
  list-style: none;
  
  -ms-box-orient: horizontal;
  display: -webkit-box;
  display: -moz-box;
  display: -ms-flexbox;
  display: -moz-flex;
  display: -webkit-flex;
  display: flex;
}

.flex-item {
  background: tomato;
  padding: 10px;
  border: 5px solid red;
  
  color: white;
  font-weight: bold;
  font-size: 2em;
  text-align: center;
}

.flex1 { flex: 1 1 20em; }
.flex2 { flex: 2 2 20em; }