Why do you want to sum the Max Principal stress from two copies of the model? You will get double the stress.

A common workflow is to have one linear model with two or three orthogonal loads where the model takes a long time to solve.  An example is a 1G acceleration load that is applied in the X, Y and Z direction, one load in each of three copies of the Static Structural model.  Then results for an acceleration load in any direction of any magnitude can be quickly created using Solution Combinations.

In Mechanical, right click on the Model branch of the Outline and Insert a Solution Combination. Then linear combinations of the three load cases can be calcuated from the solutions of the unit load cases without needing to solve again, so these results are available immediately.