Read the instructions..
While yes it sounds simple IMO having to run 2 PCM's is a PITA. As far as the tach working I'm not sure about that. The emulator from Dakota Digital I'd heard was the easy way.
I'm honestly not sure why the shop that worked on GotMud rig couldn't figure out how to get the gauges working since as you said it really isn't hard.
I have been treating the Jeep PCM like its a BCM. So right now it runs my gauges of course and I am trying to figure out how to use it to cycle the AC clutch and where it will kick on an electric fan like a XJ would. I might just put an XJ PCM in its place to have the electric fan kick on from the AC request. Also going to take all the cruise control stuff off of a spare TJ I use for beating on in the woods without much regard for its safety or mine and the Jeep PCM should be able to run it since it sees a tach, speed signal and the other needed signals for the factory cruise to work.