Note: Posted this answer because OP later stated in comments that they need to select the last two elements, not just the second to last one.
The :nth-child CSS3 selector is in fact more capable than you ever imagined!
For example, this will select the last 2 elements of #container:
#container :nth-last-child(-n+2) {}
But this is just the beginning of a beautiful friendship.
#container :nth-last-child(-n+2) {
background-color: cyan;
}<div id="container">
<div>a</div>
<div>b</div>
<div>SELECT THIS</div>
<div>SELECT THIS</div>
</div>