I can't recomend to put them together if the tank is not really big or have enough hidding places since they are "anthropofagus" and they will kill each other!
Once they eat one of them their instinct get alive and you could loss any other as did happened to me in the past. So, be carefull cause their are quite agressive if they feel there is not enough room for them!
THOUGH, if they are truely hungry they will strip to bare bones the weak one as soon as they can!
They don't play, they are not friendly anyway, they can be mercyless and really cruel, they simply eat almost anything if hungry and will do anything to survive! It's their nature.
...i think Caribe is the most beautifull and aggresive when hunting in group, i live in Venezuela and i have seen what they can do to any creature (even another Caribe) if they are TRUELY HUNGRY...they will strip ANY creature to bare bones in seconds!, THOUGH, it can be a dangerous pet or toy!!!.
My Caribe eat at least 2 fish daily and it takes less than 30 seconds to do that!
He killed one of his brothers after being together for almost a year...now i have my piranhas in different tanks.
And remember...Caribe (Notatus), Red Belly (Nattereri) and Piraya ARE THE ONLY TRUE PIRANHAS!