Depends what you're trying to make. If you actually want to make a beer that resembles coopers then PoR and as Kev says - stick with single bittering addition.
Amarillo is lovely but it will be very different from coopers, even as a bittering addition. Contrary to what many think, bittering additions aren't always neutral.
If you're happy to go outside the coopers replication then amarillo and cascade work very well together - will just be a different beer. Not sure how they go with PoR though (only used PoR twice and one is still fermenting - neither mixed with amarillo or cascade).
I agree with manticle, although I love Amarillo, I would go the Cascade as the 25% bittering fill in. And the other key with CPA is getting you yeast pitch rates and fermentation temps correct; too bready one way and way too banana the other. Although you want a slightly more bready taste, not allowing it to go too much that way is the trick to the perfect hot days beers
For such a simple beer it is so rewarding to hit it correctly.
QldKev