A polishing method includes simultaneously polishing two substrates, a first substrate and a second substrate, on the same polishing pad. A default overpolishing time is stored and an in-situ monitoring system monitors the two substrates. The in-situ monitoring system further determines a first polishing endpoint time and a second polishing endpoint time of the first and second substrates, respectively. The polishing method further includes calculating an overpolishing stop time where the overpolishing stop time is between the first polishing endpoint time plus the default overpolishing time and the second polishing endpoint time plus the default overpolishing time. Polishing of the first substrate is continued past the first polishing endpoint time and polishing of the second substrate is continued past the second polishing endpoint time. Polishing of both the first substrate and the second substrate is halted simultaneously at the overpolishing stop time.