Edit in JSFiddle

<ul id="first">
  <li>1</li>
  <li>2</li>
  <li>3</li>
  <li>4</li>
</ul>

<ul id="second">
  <li>1</li>
  <li>2</li>
  <li>3</li>
  <li>4</li>
</ul>

ul {
  display: flex;
  width: 400px;
}
li {
  width: 50px;
  height: 50px;
  list-style:none;
  border: 1px solid #ddd;
  background: #eee;
}
li + li {
  margin-left: 10px;
}

ul#second {
  flex-direction: column;
}

ul#second li+li {
  margin-top: 10px;
  margin-left: 0;
}