This may later be done by the Juventus. You're right, there's no way De Gea and Szczesny will be in the same place. I did not realize that the De Gea 4 - 5 year Contract reached 100 million.
I could not imagine why they would buy De Gea or why United wants to sell him. They're not that desperate for money and I believe there is no other good goalkeeper for United. Szczesny is not that old either, still a good goalkeeper for Juventus.
Instead of buying a goalkeeper, strengthening their other areas or youth talents might be better.