Hi Phil,
I queried this with XC themselves and their response indicated that this would need some heavy modding to cart.php (think it involved writing a new SQL query).
I actually ended up purchasing Jon's ezUpsell mod (
http://www.websitecm.com/store/customer/product.php?productid=84). Can't comment on the effectiveness of the mod itself (as we're not live yet), but it was easy to install and didn't cause any problems upgrading from 0.13 to 0.14.
If you want to see it in action, try and buy some 'Evening Invitations' on my test site (
http://www.ocpx.com). ezUpsell is 'Step 2'.
If I get a chance, i'll post XC's full response. If you don't fancy messing around under the covers, go for ezUpsell.
Cheers,