[danik.js]

<div id="block1" onMouseOver="hoverClass('block2',1);" onMouseOut="hoverClass('block2',2);">текст1</div>
<div id="block2">текст2</div>
<script>
var block1 = document.getElementById('block1');
var block2 = document.getElementById('block2');

block1.onmouseover = function(){
    block2.className = 'active';
}
block1.onmouseout = function(){
    block2.className = '';
}
</script>

или, если нужно тупо менять css, то проще в тыщу раз:

<div id="block1" onMouseOver="hoverClass('block2',1);" onMouseOut="hoverClass('block2',2);">текст1</div>
<div id="block2">текст2</div>
<style>
    #block1:hover + #block2{
        color: red;
    }
</style>