If you own two Fire Phoenix, and feed them both, the bonus damage will stack.
But it is important to understand how that type of stacking works.
Using simple numbers just to illustrate
Your base damage is 1000
Your damage with AWs is 2000
If you feed your fully evolved Fire Chicken you get a 50% bonus
That is added to your base damage so 50% of 1000 adds 500.
A second Fire Chicken that gives a 25% bonus would also add to the base, thereby 250 damage.
The temp buildings that you craft work the same way.
Everything adds to the base.
And it all helps, the feeding is worth it.
You just need to understand that it is not Base + 50% = x +25% =xx + 50% = xxx