Cluster-field segregation and determination of physical parameters of the open clusters NGC 1750 and NGC 1758 were performed from CCD and photographic photometric and astrometric surveys previously published by the authors. Membership probabilities were computed to a brightness limit of R=15mag through a non-standard method in a fully non-parametric approach, using proper motions, positions and photometry. This study allowed several parameters for NGC 1750 and NGC 1758 to be determined, including position, size, density profile, absorption, distance, age, luminosity function and mass. The common colour excess of both clusters is E(B-V)=0.34mag. NGC 1750 is slightly younger (log(t)=8.3), closer (V-M_v_=9.0mag) and less concentrated than NGC 1758 (log(t)=8.60 and V-M_v_=9.4mag). We also conclude that NGC 1746 is an erroneous assignation (not corresponding to any real open cluster), and that NGC 1750 and NGC 1758 are overlapping clusters, but they do not constitute a gravitationally bounded system.