All statements regarding absolute positioning are correct. People failed to mention, however, that you need position: relative on the parent container.
#container {
position: relative;
}
#num1,
#num2 {
position: absolute;
left: 50px;
}<div id='container'>
<div id='num1'>1</div>
<div id='num2'>2</div>
</div>Depending on which element you want on top, you can apply z-indexes to your absolutely positioned divs. A higher z-index gives the element more importance, placing it on the top of the other elements:
#container {
position: relative;
}
#num1,
#num2 {
position: absolute;
left: 50px;
}
/* num2 will be on top of num1 */
#num1 {
z-index: 1;
}
#num2 {
z-index: 2;
}<div id='container'>
<div id='num1'>1</div>
<div id='num2'>2</div>
</div>