As long as they don't have the same active ingredient you will not be overdosing. However, most meds are not 100% safe for fish, so more meds means more possible problems, but if the other option is definite problems with illness in the fish then go for it. I do combine meds when I am not certain what type of pathogen it is or when the most likely med for a particular type of pathogen isn't working.
My preference for first option meds are Melafix and Pimafix. Pimafix is great for fungus and Melafix is great for bacteria and for helping the fish heal faster (and it actually does, not one of those fake claims like stress coat or salt). If I remember correctly though Melafix can mess up your nitrifying bacteria so only use it in a QT with lots of water changes.