Wow......well, based on the pic. ...if you ever did hit the brakes in a panic stop or worst case collided with something, the only thing that would worry me would be the tongue coming through the front od the trailer if the front dolly is facing forward. That would be alot of force on such a small area and would surely come right through the front of the trailer.
UPS should design a jig that secures the front dolly to the front of the trailer floor. What you want to minimize is momentum if they start rolling.