I asked the support an they said it is not possible right now. The issue is:
We offer related products on the booking confirmation page. E.g. a customer buys product A (Train Ticket) and we offer him on the booking confirmation an additional hotel for the target city, or an insurance. If the customer decides to by the add-on product, we want to track this into the same shopping cart as the first purchase (to see the second purchase as related product in the cross sell report). Unfortunately this is not possible. Omniture would filter the second purchase if I would use the same purchaseID of the first product. The products are not related in cross sell report if I use another purchaseID. Too bad ;(
