A link to the website would prove to be helpful in order to see the source code however, I will assume that the problem you are experiencing is due to inserting a form between another form in the html.
Let's take** tpl_product_info_display.php** for example. When you add an additional form above the code
<!--bof Form close-->
</form>
<!--bof Form close-->
found on lines 236 to 238, a conflict in the html is created since the two forms will utilize the same id.
What I normally do when modifying the file tpl_product_info_display.php is move the following
<!--bof Form start-->
<?php echo zen_draw_form('cart_quantity', zen_href_link(zen_get_info_page($_GET['products_id']), zen_get_all_get_params(array('action')) . 'action=add_product', $request_type), 'post', 'enctype="multipart/form-data"') . "\n"; ?>
<!--eof Form start-->
found on lines 18 to 20 to immediately above
<!--bof Add to Cart Box -->
found on line 88 and also move the code
<!--bof Form close-->
</form>
<!--bof Form close-->
immediately below
<!--eof Quantity Discounts table -->
found on line 155.
In this manner, any cross sell form within your html will have its unique id as well as the Add to Cart form for the main product.
