Well, I made the pies and took them out when the rhubarb was tender, though they were bubbling on the edges but not the center. There was no raw cornstarch flavor, and the filling was quite thick.
In the interests of science, I should note that I didn’t quite follow the recipe. I had more rhubarb than cherries, and the cherries were canned, unsweetened pie cherries. I kept the total weight at 1 lb. per pie. I used the liquid from the can for the extra liquid(1/2 c for the two pies). This may have been less liquid total that the recipe is planned for, because there wasn’t enough filling. Some chunks of raw rhubarb stirred in during pie assembly brought the volume up. These were the chunks that held up best during the baking. They did go from crisp to very tender in 5 minutes.
Anyway, the pies were very well received, and the birthday party was a success. The food was good enough to lure people into staying at the table, enjoying each other’s company for a very pleasant evening.