[monolithed]

ale19,
А думать кто за вас будет?

<script>
window.onload = function() {
    var div = document.getElementById('div').children,
        i = div.length;

    while(i--) {
        if(i % 2) {
            var div2 = document.createElement("div");
            div2.className = 'div2';
            div2.innerHTML = 'div2';
            div[i].parentNode.insertBefore(div2, div[i].nextSibling)
        }
    }
};
</script>

<div id="div">
    <div class="div1">div1</div>
    <div class="div1">div1</div>
    <div class="div1">div1</div>
    <div class="div1">div1</div>
</div>