GLKit combines two textures

I am trying to convert OGL ES 1.1. code for GLKit. GLKit offers a couple of texture slots:

• texture2d0
• texture2d1

Each texture has an env mode:

• GLKTextureEnvModeReplace,
• GLKTextureEnvModeModulate,
• GLKTextureEnvModeDecal

Usually you leave texture2d1 empty and just install texture2d0. I suggested - from reading Apple documents - that 2d1 is for mixing / combining / modifying textures. Since GLKit "just" sits on top of the shaders, and for each shader it is standard to have a pair of texture slots - an "input" slot that represents what is already on the material, and a "modification" slot that represents the material that has the shader will be used as a parameter for material modifications.

But that does not work.

I tried:

self.baseEffect.texture2d0.envMode = GLKTextureEnvModeReplace;
self.baseEffect.texture2d0.target = GLKTextureTarget2D;
self.baseEffect.texture2d0.name = [earthTextureDefault name];

self.baseEffect.texture2d1.envMode =  GLKTextureEnvModeModulate;
self.baseEffect.texture2d1.target = GLKTextureTarget2D;
self.baseEffect.texture2d1.enabled = TRUE;
self.baseEffect.texture2d1.name = [textureClouds name];

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