How to fade one texture into the other

So basically I have a sky model and I want it’s texture to fade from this texture (default one):

To this texture:

Let’s say that fading isn’t really the problem here (I will probably use a function lerp) but the main question is how to add the secondary texture to the model setting it’s transparency to 0 then using my function lerp I will decrease the primary texture’s transparency and increase the secondary texture’s transparency, so what function method should I use to accomplish the transparency goal?

You could apply both textures to the same model, and use combine mode CMInterpolate to interpolate between the two.

This is explained on this page:

On the second texture stage, you would set a CMInterpolate mode to interpolate between the previous and current texture stages based on some constant factor (such as the alpha scale).