The idea is that the word "America" in FAA is limiting so Pixels was born as the main site. Pixels offers more products. FAA sticks mostly to fine art substrates although there are some "gift shop" items mixed in.
Each attracts its own audience and sales come in from both. From the artist's standpoint, you only have to upload to one or the other and your work is offered on both as well as many more mirror sites such as Designer Print which caters to interior design professionals and some other clone sites with different URLs.
If you look closely at the two sites you'll see slight cosmetic differences but each site uses the same backbone, databases etc.
