The second ground is just a place to connect the power supply ground if you don't connect it to the chassis. If you look at the board, they are just tied together.
In most applications, people connect ground in and Vin to the battery, then you have a power supply with Vout and ground out, you connect them to the SDC. In your case ground is ground and you don't have to connect the other lug to anything if you don't want to.
Ground in and out are interchangable.