In this paper, we theoretically investigate 21 projectile-target combinations of stable isotopes for the pre-synthesis parameters
and production cross sections of the unknown superheavy 309,312126 nuclei. It is found that 61Ni + 248Cf and 64Ni + 248Cf
combinations are the best candidates for the synthesis of the 309,312126 isotopes due to their largest cross sections of 0.03
and 3.6 pb, respectively. Besides, the results in the present and previous works indicate that the uncertainty, from pb to zb,
in the synthesis cross sections of superheavy nuclei should be narrowed for a decision of measurements using the presently
available facilities. This study, thus, provides valuable data for the synthesis of 126th element.