A polishing method includes simultaneously polishing a first substrate and a second substrate on the same polishing pad, storing a default overpolishing time, determining first and second polishing endpoint times of the first and substrates with the in-situ monitoring system, determining a difference between the first and second polishing endpoint times, and determining whether the difference exceeds a threshold. If the difference is less than the threshold, then an overpolishing stop time is calculated and polishing of the first substrate and the second substrates is halted simultaneously at the overpolishing stop time. If the difference is greater than the threshold, then first and second overpolishing stop times that equal the first and second endpoint times plus the default overpolishing time are calculated, and polishing of the first and second substrates is halted at the first and second overpolishing stop times, respectively.