Grounding between enclosures in a closed system
Since you are showing analog and signal grounds being connected between the two boxes, connecting the power grounds together is not a great idea. You are inviting ground loops. 

That said, if your boxes are at all separated physically, and your interconnecting signals are low voltage/ high impedance (10 volts, a few k), you will be very well advised to use differential transmission for your interbox signals. Particularly when dealing with 28 volts systems, the IR drop on the power lines can produce significant voltage offsets. Since your 28 volt return is labeled ground, if your boxes are likewise grounded you need to be extremely careful of your internal ground routing. Make sure that your signal and digital grounds are isolated from case ground by several k.