Nyquist wavelength-division-multiplexed coherent optical superchannel systems closely pack a set of rectangularshaped channels to realize high-spectral-efficiency transmission. In long-haul fiber links, dividing each of these channels into multiple subbands, each a few-gigahertz wide, can increase tolerance to fiber nonlinearity. However, these low-bandwidth subbands will be sensitive to laser drift, which could cause them to overlap, creating interchannel interference (ICI). In this paper, we propose the application of pairwise precoding across the middle and edge subbands to improve their ICI tolerance. The proposed scheme is verified in superchannel experiments, and shows about 1.5-dB Q2 improvement after 3840-km transmission for QPSK modulation and 1-dB Q2 gain with 16-QAM modulation after 800-km transmission.