Make an override copy of includes/templates/template_default/templates/tpl_search_header.php and replace this line
Code:
$content .= zen_draw_input_field('keyword', '', 'size="6" maxlength="30" style="width: 100px" value="' . HEADER_SEARCH_DEFAULT_TEXT . '" onfocus="if (this.value == \'' . HEADER_SEARCH_DEFAULT_TEXT . '\') this.value = \'\';" onblur="if (this.value == \'\') this.value = \'' . HEADER_SEARCH_DEFAULT_TEXT . '\';"') . ' ' . zen_image_submit (BUTTON_IMAGE_SEARCH,HEADER_SEARCH_BUTTON);
with
Code:
$content .= ' OR ' . zen_draw_input_field('keyword', '', 'size="6" maxlength="30" style="width: 100px" value="' . HEADER_SEARCH_DEFAULT_TEXT . '" onfocus="if (this.value == \'' . HEADER_SEARCH_DEFAULT_TEXT . '\') this.value = \'\';" onblur="if (this.value == \'\') this.value = \'' . HEADER_SEARCH_DEFAULT_TEXT . '\';"') . ' ' . zen_image_submit (BUTTON_IMAGE_SEARCH,HEADER_SEARCH_BUTTON);