The application of digital subcarrier multiplexing (DSM) in elastic optical networks necessitates advanced optical performance monitoring (OPM) for various transmission impairments owing to its flexible spectral allocation and adaptive transmission capability. In this work, we propose a two-interleaved pilot-tones (TIPTs)-assisted OPM approach that enables simultaneous estimation of multiple transmission impairments and system parameters, including frequency offset (FO), polarization rotation (PR), chromatic dispersion (CD), transmitter (Tx) and receiver (Rx) individual laser phase noise (LPN), and modulation format (MF). With optimal TIPT parameters, the simulation results reveal that the proposed OPM scheme can simultaneously achieve an FO estimation error below 35 MHz, a CD estimation error within 120 ps/nm after 2000 km standard single-mode fiber (SSMF) transmission, and robust PR tracking under 100 krad/s polarization variations. In particular, the explicit separation of Tx/Rx LPN effectively eliminates equalization-enhanced phase noise (EEPN), while the embedded amplitude-modulated TIPTs enable nearly 100% MF identification (MFI) even under optical signal-to-noise ratios (OSNRs) as low as 20 dB. The proposed OPM scheme is experimentally validated when the 80 GBaud DP-16QAM DSM signals are transmitted over a 1600 km SSMF. Those results confirm the accurate and robust operation of TIPTs-assisted OPM for future DSM-enabled elastic optical networks.