It all boils down to the details. Not only would the two lasers have to be 2 Hz away, but the line widths would have to be much less than 2 Hz. Lasers aren't actually monochromatic, just very narrow band. I haven't run the numbers, but you would have to operate said lasers very cold to get thermal noise that low, and thermal is just one of the sources. Think of things like doppler on the end mirrors from vibration and piezo effects. That one part in ten to the 14th is tough. It may be possible theoretically (not even sure of that), but in practice it just can't be done.