I like that you have sand in your tank. That's perfect for the dwarf puffers.
I've had my 2 dwarf puffers for 2.5 yrs now in a 12 gallon (they're so awesome) and they still manage to fight over the food. Not fight fight but they like their space. They fought a lot more when they didn't have enough hiding spaces so I had to add some. I'd probably add some taller plants to the tank and try to make some hiding spots in case they need to get away from each other. These guys will definitely grow on you