A chemical mechanical polishing system includes a platen to support a polishing pad, two carrier heads configured to hold two substrates against the polishing pad at the same time, two actuators to sweep the two carrier heads laterally across the polishing pad, an in-situ polishing monitoring system including a two current sensors to sense two currents supplied to the two actuators and generate two signals, and a controller to receive the two signals and independently detect a two endpoints for the two substrates based on the two signals.