I added support for attributes in this mod.
I not tested 100% but I think there are no bugs.

For update look in catalog/includes/application_top.php for:
$cart->add_cart($HTTP_POST_VARS['buy_tinn_add'], $cart->get_quantity(tep_get_uprid($HTTP_POST_VARS['buy_tinn_add'], $HTTP_POST_VARS['id']))+1, $HTTP_POST_VARS['id']);

Replace for:
$cart->add_cart($HTTP_POST_VARS['buy_tinn_add'], $cart->get_quantity(tep_get_uprid($HTTP_POST_VARS['buy_tinn_add'], $HTTP_POST_VARS['id']))+1, $HTTP_POST_VARS['id_b']);

Replace catalog/includes/modules/2gether.php




More...