<div class = 'container'>
<ul class='list'>
 <li class = 'item'>1</li>
 <li class = 'item'>2</li>
 <li class = 'item'>3</li>
 <li class = 'item'>4</li>
 <li class = 'item'>5</li>
 <li class = 'item'>6</li>
 <li class = 'item'>7</li>
</ul> 
 
 
 <ul class='list'>
 <li class = 'item'>1</li>
 <li class = 'item'>2</li>
 <li class = 'item'>3</li> 
</ul> 
</div>

.container {
  height: 200px;
  overflow-y: auto; 
  background-color: lightcoral;
  display: flex;
 
}

.container > :first-child {
 margin-right: 20px;
}


.list {
  list-style: none;
  display: flex;
  flex-direction: column;
  justify-content: center;
  margin: 0;
  padding: 0;
}


.item {
 flex-shrink: 0;
  height: 40px;
  width: 100px;
  border: 1px solid black;
}