I do a lot of Amarillo based beers, I also have mixed them with Cascade at all stages of boil and dry-hopping. My opinion is that Amarillo and Cascade are similar, but do not complement each other as well as expected. I get too much grapefruit/citrus from the two together as flavor hops.
Amarillo at 20 is great, aroma, taste and bittering. I would choose one or the other for your "featured" hop. If you can get your hands on the Green Flash HopHead Red, it is late hopped and dry hopped with Amarillo-that is a fantastic beer.
As remilard mentions, I like Amarillo with Simcoe too.