Low-field electric control of magnetic phase transitions is critical for the development of energy-efficient spintronic and non-volatile memory technologies. Yet, the weak magnetoelectric coupling in most known two-dimensional multiferroics hinders their practical implementation. Here, using crystal structure prediction and high-throughput first-principles calculations, we identify four previously unexplored bimetallic thio(seleno)phosphate multiferroics, XMnP₂(S/Se)₆ (X = Cu, Au), all exhibiting robust in-plane spontaneous polarization—contrasting with the predominantly out-of-plane behavior in this material family—which effectively mitigates depolarization effects. In particular, CuMnP₂Se₆ hosts two stable C₂-symmetric ferroelectric phases with opposite in-plane polarizations and distinct magnetic orders. Remarkably, an electric field as small as ~0.001 V/Å can simultaneously reverse the polarization and induce an antiferromagnetic-to-ferromagnetic transition. The associated barrier is exceptionally low ( ~49 meV/f.u.), yielding a sizable magnetoelectric coefficient of ~0.04 G ⋅ cm/V. These results highlight a viable strategy for realizing electric-field-driven magnetism in intrinsic two-dimensional multiferroics under experimentally feasible conditions.