Hi, I took a look at this and though Patran will allow you to average both layers whilst doing (e.g.) a fringe plot I could not see a way to do it for the report and all load cases.
Can you rerun the analysis and request Element Force output (FORCE(SORT1,REAL,BILIN)=ALL) as this will probably give you what you want directly as the force at the middle. Note this is force per unit length so dividing by the shell thickness will give you an equivalent stress value.