There is no way you can out grown RBP in 40gallin to 10" infact not even 75gallon or 90gallon...
if you want to out grow RBP get atleast 125gallon or bigger. The bigger tank the better for them.
2 piranha in same tank are bad ideas. Piranha do well with group of 3 and/or more. If you have 2 piranha in one tank their will fight over territories and aggression.
I would get just one RBP for your 40gallon.
Also having BIG tank are not the only thing you can do to out grow RBP to 10".
for sample: for you have 200gallon with 3 RBP in there and you are trying to grown them up to 10" is not going to get there if you doesn't know how the proper care them.
Here is some tips to out grow piranha in short term and grow fast.
1. tank size, the bigger tank you can get the better for them (more space to exercise)
2. Water change, atleast once a week about 30-50% with vacuum the gravel.
3. Water quality, keep your ph, kh, etc...in good # is one of the most important thing to grow piranha
4. Diet, feed piranha variety diffferent diet. Don't over feed them or don't feed them small portion.
small 2-3 time a day
medium 1-2 time a day
adult once a day or every other day with good portion of diet.
It's all combo with well maintain to keep piranha healthy and grow big and fast. Not just tank size, not just food, not just enviroment, it's ALL together.