More than one pop up is not coming in IE

I have the following html

<fo开发者_如何学运维rm action="http://www.test.com" method="post" name="form_test"    target="" >
    <input type="image" src="loader.gif"  id="icon" alt="image" onclick="popup();"/>
</form>

<div id="ij">
ghgfhg
</div>

<script type="text/javascript">
function popup(){
    window.open ("http://www.google.com","one");
    window.open ("http://www.yahoo.com","two");
}
</script>

And when I click the image button I get two pop ups, one for google and one for yahoo in every browser except IE.

In IE, only one popup with 'google', is coming.

And if I allow popup blocker settings in IE brower for the site, where the page is located, then I get two popups.

So let me know what is the reason in IE.

---

The popup blocker blocks every popup except the first.

http://www.microsoft.com/austria/windowsxp/sp2/sp2_popupblocker.mspx

explains in the last paragraph parts of how it determines whether to block.