Your question is a bit weirdly phrased. Do you mean that if you do a texture lookup but only use let's say the alpha component of the returned float4 it gets cheaper? if so, highly doubtful, since the function returns a float4 and you merely discard parts of the return value. Since all modern shader compilers are SSA based you could potentially argue that if the ISA supports it the compiler could optimize it to a specific single channel fetch, but I don't think any vendor does that.

In any case, a texture lookup is incredibly cheap, especially once the texture is in the GPU cache. You can totally get away with two texture fetches per fragment in order to do your effects. That's pretty much what the whole industry does anyway.