You have to be careful with combination load cases, depending on the components (what base cases are combined), whether or not the system is non-linear, and the combination method empolyed (scalar, algebraic, absolute) - some results may not be applicable.

If you want to ensure that the combined displacement doesn't exceed 20 mm, why not use an OPE case which includes all SUS and OCC loads (also thermal loads) instead?

(Displacements are not computed from stresses, rather it is the other way around. The FEA system is solved for the displacements. From the displacements the element's forces and moments are determined. From the forces and moments, the Code equations are applied to compute the stresses. To see the differing effects of the combination methods {scalar, versus algebraic, versus absolute}, see the help text in the Static Load Case Editor - there are numeric examples there.)