mova-
I agree with deanc's references. Basically, you have to design for the worst possible combination of pressures: One side may have internal pressure while the other is exposed to a vacuum so the loads would combine.
Most often you would see a multi chambered vessel with a 2:1 SE intermediate head attached similarly to what is shown in Fig. UW-13.1(f). In such a case, you must check the head for both internal pressure (pressure on the concave side plus vacuum on the convex side) and for external pressure (pressure on the convex side plus vacuum on the concave side). You would generally use the higher of the two chamber's design temperatures for both the internal and external pressure checks.
jt