Good point, these molecules are after all 3D struc

Good point, these molecules are after all 3D structures and flexible, one can see how tacking on another drug could change the binding properties of the first, and yes, it's simpler to just give the two drugs. Perhaps down the road someone with deep pockets will give conjugation a shot for some indication where receptor targeting makes sense.
Thanks to you and Ohm for the knowledgeable replies.