In no place on this chart, does API combine those two although they do indeed show many combos. So for whatever reason they do not suggest combing API General Cure and Melifix. Look at the chart which may give you direction, but I wouldn't combine those unless someone chimes in that it's okay to do.
Regardless, what are you trying to cure? Parasites, bacteria, fungal infections, etc.? If you have pictures that you can post, I'm certain it will help get responses.
Also: post the type of fish and the water parameters (tank size, ammonia, nitrite, nitrate, pH, temps, and hardness.) Some fish don't do well with certain meds and water params can affect med use as well.