Soy sauce and fish sauce are not often found in combination. I've seen the occasional comment recommending a substitution of soy sauce for fish sauce, but it seems to me that if you want to cook Thai (say) and can't find fish sauce, you should pick another cuisine to cook. Both sauces add salt to a meal, but their other aspects are so different that they are not interchangeable.
I recently got some of Lonely Planet's World Food guides. (My series overview for Food, Bound can be found here.) Though these books are mainly travel guides, they do include recipes. I was reading the World Food guide to Thailand when I stumbled across a recipe for "Kung Phat Khing" or "Prawns Stir-fried in Ginger." It looked tasty and simple, and it called for a combination of soy sauce and fish sauce. According to the guide, it's a Chinese-style dish adapted to Thai-style cooking. Because it looked so delicious and simple, I grabbed the book and took it into the kitchen for a really off-the-cuff brunch.
Stir-fry some chopped garlic and ginger in vegetable oil until golden. Then add three tablespoons chicken stock, one and a half tablespoons fish sauce, one tablespoon soy sauce and half a teaspoon sugar. Cook until bubbling, then add eight large shrimp (shelled), eight straw mushrooms and two scallions (sliced into two and a half inch lengths). Add a sliced hot chili at this stage for more heat, if you like. Stir-fry until the shrimp are opaque, then serve.
I made minor substitutions of porcini soaking water for the chicken stock and sliced button mushrooms for straw mushrooms. The final result was very tasty. The combination of the fish sauce, soy sauce and porcini water gave the sauce an extra savor of a quality that I usually find in beef stir-fries.