A split digital backpropagation (DBP) scheme for digital subcarrier-multiplexing (SCM) transmissions, denoted as SSDBP, is proposed and studied in both experiments and simulations. The implementation of the SSDBP is split at the transmitter and the receiver, leveraging existing chromatic dispersion (CD) compensation blocks to reduce complexity. We experimentally demonstrate that the SSDBP, with a complexity reduction up to 50% compared to the original receiver based SCM-DBP, can achieve a nonlinear compensation Q2 gain of 0.7-dB and 0.9-dB for 1920-km and 2880-km 34.94-GBd single channel PDM-16QAM transmissions, respectively. The maximum reach can be extended by 31.6% using 2-step SSDBP with only 27.5 complex multiplications per sample. Meanwhile, using 3-step SSDBP, the reach extension can be increased to 40.8%. The benefit of implementing part of SSDBP at the transmitter is experimentally validated with 0.1-dB Q2 improvement at 4-dBm launch power. We also numerically investigate the impact of the digital-to-analog converter (DAC) resolution and fiber parameter uncertainties on the nonlinear compensation performance of the SSDBP.