Hello,
I don't know how to combine two normal maps together in MaterialX, is there any node to do this? I've seen some examples of how to make a mathematical implementation in Blender that could probably be translated into materialX, but I want to make sure if this is the only way to make it work.

Thanks,
Dawid

mtlx is missing the quaternion and matrix math operations that would make this doable in the usual way. However, you could implement the component-wise scalar multiplications and additions that compose the higher level matrix math.