This gauge is based (a Remix) on the Dualstrustion nozzle leveling Fixture by einer_designs: http://www.thingiverse.com/thing:23013. That Thing is meant for leveling extruder nozzles on a Rep 1 Dual, but I found that it would not work for aligning nozzles on the Rep 2X, either the OEM mounting block, or Carl Raffle's new cooling block. This can be used for either design. Nozzles are adjusted until both are touching the surface of the tool, while the mounting block is held tightly against the tool.
This Thing was created based on the example of einer_designs Dualstrustion nozzle leveling Fixture. Since I could see that it would not work for what I had to do to upgrade my MakerBot Replicator 2X, I Remixed it to meet those needs. You should print this Thing in ABS (the main material used with the Rep 2X) and scale it 101.5%. This will allow for shrinkage. I would lay the design on its side (so that no support is needed) and for a clean surface with Kapton tape, do not use a raft. Any slight flash should be removed if it might interfere with accurate alignment to the mounting block.
In use this tool should give you a reference for aligning both extruder nozzles the same distance from the gantry when they are mounted on the aluminum mounting block, and it in turn is mounted on the gantry carriage. Any slight adjustments after that can be achieved by using fine shims under one end of the mounting block or the other. Care should be used to make sure when the nozzles are next used in leveling the build plate, to make sure that during the beginning of the leveling process, the build plate doesn't crash into the nozzles. The leveling screws should be turned in all the way before this is done, and if necessary, a shim may be used to activate the stop switch earlier in the build plate's movement.