My opinion is: turtles and tortoises that evolved on different continents from each other should never be housed together (not to mention that the sulcata's bulldozer-like marching, even when a baby, may harm the more fragile baby box turtle).
You are asking for trouble not only different feeding requirements, but that sulcata is going to outgrow that boxie by leaps and bounds, and it may end up hurting it or worst