box-shadow support multiple commas which mean this is possible and can be done like below:
box-shadow: 2px 2px 3px #888, -2px 2px 3px #888;
The first group bring shadow to the right & bottom while the second group bring shadow to the left (by using negative value) & bottom.
The complete code for cross browser support is:
-moz-box-shadow: 2px 2px 3px #888, -2px 2px 3px #888;
-webkit-box-shadow: 2px 2px 3px #888, -2px 2px 3px #888;
box-shadow: 2px 2px 3px #888, -2px 2px 3px #888;