The USB voltage feeds into the board downstream of the VIN regulator, at the 5v stage.I think you can power the 2nd board by running a wire between the 5V connection on the socket strips,
In this case the "trusted 5v source" can be the 5v pin on the other Arduino.
Am I interpreting the 200mA note correctly? It sure seems low.
The issue with putting diodes in parallel is that as they heat up their resistance decreases. As a result, that diode ends up taking on more current then the other diode, resulting it in heating up even more. As you can probably see this cycle will cause thermal run away causing the diode to eventually burn if you give it enough current.