The hyperfine structure of the two isoelectronic molecules H2CN and H2CO+ in their electronic ground state (X(2)B(2)) is studied. The influence of the atomic orbital (AO) basis sets, of the correlation treatment, and of the equilibrium geometry on the obtained hyperfine properties is investigated. It is found that the multireference double excitation-configuration interaction (MRD-CI)/B-K treatment in which an MRD-CI wave function is correlated by a modified B-K method yields equivalent results to quadratic CI [QCISD(T)], coupled cluster single doubles [CCSD(T)], or Brueckner doubles [BD(T)]. Uncertainties in the equilibrium geometries are found to be the major source for discrepancies between theoretically and experimentally determined isotropic hyperfine coupling constants (hfccs). For the heavier centers, the calculated values of the isotropic hfccs agrees nearly perfectly with experimental values (approximate to 1%-2%). The calculated values for the hydrogens are too low, but using the equilibrium structure suggested by Yamamoto and Sato [J. Chem. Phys. 96, 4157 (1992)], the best estimate deviates by less than 3%.