I have zero experience of dwarf puffers, only their relatives, the slightly larger red-tail puffers, so I'll be interested to see what people say in answer to your question.
Keeping multiple puffers of most species is a gamble, and I think you'll find for every one person for whom things work out fine, there's another who's had nothing but trouble. One idea might be to put a mirror beside the tank, and see how the fish react. Sometimes fish don't "get" it's a mirror, and will react to the mirror image of themselves as if its another fish. If the fish are mellow, then maybe it's worth a shot, but if the fish go bananas, then you'll have your answer.
Cheers,
Neale