#X { 
  position: absolute; 
  top: 50px; 
  left: 50px; 
  background-color: #99cccc;
  height: 100px; 
  width: 50%;
  border: 1px solid black;
}

#Y { 
  position: absolute;
  background-color: #cc99cc;
  height: 100px; 
  width: 50%; 
  border: 1px solid black;
}