I guess I am a little confused by the question. He should already have the two. Does he mean to imply that one of them is not working? And where could he get a replacement part?
If he wants to know what he can power with them? Then the answer might be almost any 12V-DC accessory that wont exceed the amp. limitations of the circuit.
Works great to plug in a sprayer pump on a large tank to ride in back with a spray wand to spray weeds along the fencerows at the farm while your sister drives along the fences.
Only newer vehicles have the on board inverters. Like Eddie said you can get an inverter and just plug it into the 12V outlet. I've used a 400 watt one to charge a laptop or VHF radio, etc. Ive seen a guy with a 1st gen Explorer that ran a 2nd battery and powered a 2000W inverter for his on board air and water pump.