The reason div2 displays above div1 is because div2 is absolutely positioned. That means that div1 doesn't participate in the normal document flow, as if it was pulled out of the document. So, div2 shows up at the top, then your absolute positioning pushes div1 down to 100px.
Take the absolute positioning off of div1, then use margins or padding to move it down to the desired location. That way, the normal html rendering will place div2 below div1.
If you're forced to absolutely position div1, then you need to absolutely position div2 as well. You may need to use javascript to figure out the height of div1 and set the top of div2 appropriately.
<html>
<body>
<div id="Div1" style="position:absolute; top: 100px;">
<span>My text</span>
</div>
<div id="Div2" style="position:absolute; top: 130px;">
Test
</div>
</body>
</html>