  We develop differential measurement protocols that circumvent the laser noise
limit in the stability of optical clock comparisons by synchronous probing of
two clocks using phase-locked local oscillators. This allows for probe times
longer than the laser coherence time, avoids the Dick effect, and supports
Heisenberg-limited measurement precision. We present protocols for such
frequency comparisons and develop numerical simulations of the protocols with
realistic noise sources. These methods provide a route to reduce frequency
ratio measurement durations by more than an order of magnitude.
