at this size, im pretty sure adding more Ps would not be a great idea. they're already adult and would probably not accept any other buddys with them, even if the same size. it could work, but you have high risk of non-acceptance when rbp reach almost adults size.
if the two have been together for a while..they will be fine together.. if they are from separate tanks and didn't grow together then it's risky.
.. mine are about an inch bigger i got them around 5 inches they were still pretty big..i transfered them from my friends tank to a tank i set up and they are doing great.
The best way to add more piranhas to a tank is to take everything out of it (decors + fish), do a large water change and redocarate the tank in a different way. Then add all the piranhas at the same time... the old ones will loose their territory...