I created a solution based on the Filemaker Solutions Expense Report. In it I recreated the presentation of Start Date and End Date based upon the minimum and maximum dates entered in the two portals.
What I was finding was the dates being returned appeared to relate only to the first portal. Now I am not sure that the calculation within the Filemaker Solution is actually correct, as I experimented with some dates and found that the results were incorrect; but nonetheless had to solve my problem.
Initially I made the Start and End Date fields Calculation type fields and example unstored = Min( childA::Date ; childB::Date) but appeared to be getting results return from childA only.
After some playing around I changed the calculation to:
unstored, = Min ( Min (childA::Date) ; Min (childB::Date)) and it seemed to work. Not sure why - but wonder whether the specific Min (/Max) calculation on each portal is forcing the desired results whereas the single calculation for some reason does not. Any thought or observations welcomed.