Setting up and using the MIDI Solutions Merger is a simple process, as you simply plug your MIDI signals, whether it be keyboards or whatever, to your two MIDI inputs and you send your MIDI output to your MIDI interface. It really doesn't get all that much simpler than that, and that is really all there is to it, so a manual isn't necessary.
MIDI Solutions specializes in unique MIDI boxes and interfaces, so I trust them when it comes to things like this. The Merger is a well designed product that will do exactly as advertised. The price is reasonable and realistic, so if you need a box or two like this one for when you might have a limited amount of MIDI inputs on your interface, I wholeheartedly recommend making the MIDI Solutions Merger the first place you look. I'm really not sure about a product that I would compare with this one, as the Merger is the product that I have used from the beginning and that I will probably continue to use for a good time to come.