Got it.

That's the ticked, tomorrow I'll upload an updated version:
(The cost is the necessity of 2 tga textures)

Code:

---

matrix matMtl;
texture entSkin1;
texture entSkin2;

Technique fluid_shift
{
	Pass p0
	{
		Texture[0] = <entSkin2>;
		Texture[1] = <entSkin1>;

		TextureTransform[1] = <matMtl>;
		TextureTransformFlags[1] = Count2;

		ColorArg1[0] = Texture;
		ColorArg1[1] = Current;
		ColorOp[1]   = Modulate;
		ColorArg2[1] = Texture;

		AlphaArg1[0] = Texture;
		AlphaOp[0]   = SelectArg1;

		AlphaArg1[1] = Current;
		AlphaOp[1]   = Modulate;
		AlphaArg2[1] = Texture;

		ColorOp[2]   = Disable;
		AlphaOp[2]   = Disable;
	}
}

---