How do I separate two fish fillets that have been frozen together? Sometimes when I get frozen fish, it comes with several pieces frozen together (specifically, Trader Joe's does this with all of their frozen fish). 

Is there any good way of separating them without fully thawing?

This could also apply to other frozen things, but I'm mainly interested in fish.
If the pieces are only frozen together in a few places, you might want to try heating a butter knife up under hot water and then using that to pry them apart. This is similar to heating up an ice cream scoop. It won't work if they are really packed in together tightly though.