To make full use of the promotion potential of hydrostatic stress on the formability of sheet metal, several foundational questions met in practical application of double-sided pressure forming technique are studied in this paper. The theoretical analysis of sheet metal deformation during the double-sided pressure bulging process is carried out, and an analytical approximation for normal pressure loading path that secures out-of-plane stress ratio, γ, continuously increasing (dγ ≥ 0 holds) is presented. Utilization of the relationship between the theoretical approximation for normal pressure load and material hardening properties, a novel double-sided pressure forming method is proposed—the viscous pressure bulging (VPB) of the overlapping sheet metals, in which a highly viscous but flowable semi-solid medium is used as the soft die and the target sheet metal is deformed under the constraints from viscous medium on one side and overlapping sheet on another side. To confirm the efficiency of the proposed method, the VPB test of overlapping sheet metals is simulated by DEFORM-2D, and the experimental tests are also carried out. The results show that σ3 provided by overlapping sheet can overcome the diminishing of γ value caused by evolving strain-hardening and geometric softening effect in the post-diffuse necking stage. The experimental obtained increase of the limit dome height (LDH) of double-sided pressure bulging specimen is 12.6% compared with that of single-sided pressure bulging specimens.