Two is definitely not a good idea, and as Sue stated even one would be on limited time. Porcupines get very large and most are fairly active. Your water will become very hard to keep in check...puffers are especially messy when eating. You also have to consider the aggression of puffers. My porcupine started off very passive, but in the last months has become increasingly aggressive leaving circular marks on every fish in the tank...his new home is being prepared at the moment.