GS has a (somewhat) new feature that automatically pairs diffuse and normal maps. For example, say you skin one part of your mesh with skin1, then place that skins normal map after it (skin2). For the next segment of your model, use skin3 for diffuse, and skin4 for it's normal map.

GS will send the model as segments, rendering the first pair, and then again with the second pair, and so forth...