Pretty much no matter what you do, you will be damaging the product (either physically or otherwise) to some degree. The balance is, how much are you willing to damage the product vs. how much time and effort do you want to expend?
(Non)Individually Quick Frozen foods are a pain if you only intend to use one of the fillets at a time.
Quick and sloppy:
Pry the frozen fillets apart using a case knife or use a wedge of some sort to separate them physically without thawing.
Middle ground compromise:
A hack saw can be used to good effect. It's especially easy to make two "half fillets" by cutting across the fillets together instead of trying to separate them.
Easy does it:
Try thawing for a couple of moments in shallow cold water. Keep the fish you intend to "throw back" out of the water as much as you can to avoid thawing-refreezing-thawing damage for next time.