aristotle - 6:10 pm on Jul 17, 2013 (gmt 0)
I don't think you can simply add the existing traffic to the two sites together to predict the final traffic to the target site. Its Google search rankings could change because of the new backlinks added to its backlink profile. Also, after the first site drops out of Google's index, it will no longer be transferring any search traffic to the target site, although other kinds of traffic will still be transferred.
In my opinion the final results are hard to predict