I have not personally seen the set up he was running, but I'm not sure how having dual cases installed would make the installation stronger. The crossmember should direct the load into the frame even if there are no t-cases installed. I know that he used some spacers that might create some stress points, but I'm not sure why one vs. two cases would make a difference.
With the second case installed the drivetrain acts as a stiffener to help alleviate the out-of-plane loading on the crossmember.
If you do not have the second case installed the rear of the crossmember becomes an unsupported cantilevered beam. When it is loaded up it transferes a large bending moment right to the crossmember mounts (where it bent).
