In order to achieve a compatible solution between the divertors and the core, SOLPS-ITER simulations were performed on STEP (Spherical Tokamak for Energy Production) connected double-null geometry to investigate the possibility of using deuterium (D) fuelling puff locations as an actuator for divertor argon (Ar) retention. Significant reduction of upstream Ar density was obtained when D was puffed from outer- and inner-midplanes rather than from private flux regions, which resulted in 2-3 orders of magnitude higher Ar compression and enrichment and a factor 4 lower upstream Ar concentration to achieve acceptable target conditions. The key for the significant reduction of upstream Ar density was the induced outflow of middle charge-state Ar ions such as Ar⁷⁺. This outflow was achieved by a combination of D⁺ outflow, high collision frequency, and flipped temperature gradients around the inner midplane. As all of those are effects of the midplane puff, if enough deuterium is puffed from a midplane, one can expect outflow of middle-charge-state Ar ions that increases Ar compression and enrichment by reduction of the upstream Ar density. Further studies should be carried out to investigate if this effect remains desirable under a possible drawback such as increased upstream main plasma density.