<form name="iblock_add" action="<?=POST_FORM_ACTION_URI?>" method="post">
Текст 1: <input type="text" name="text_1">
Текст 2: <input type="text" name="text_2">
<input type="submit" class="buttom_style_reg_firm" name="iblock_submit" value="Submit"/>
</form>
<script type="text/javascript">
document.iblock_add.text_1.onchange = function(){
document.iblock_add.text_2.value=document.iblock_add.text_1.value;
}
</script>