If you are getting two instruments sounding at the same time, I assume that they are simply both getting the same data. There may be some high-tech ways to stop this, with extra boxes etc, but the simplest method is to set channels on devices. Check which devices have a setting for Midi Channels Received, most do. Some devices will by default receive on certain channels ONLY, make sure there are no overlaps. Those devices that can receive on ALL channels may need to be limited, so the available channels are shared out. You may need to change channels in some midi filef if you don't want to have all devices active.
An even simpler option is to switch certain devices OFF if they are duplicating channels.