Surprisingly, Eric, cool and cold water species will readily cross into the epilimnion for short periods of time to feed, provided the temperature differential isn't too great. As Cecil, often reminds us, it's usually the lack of oxygen that kills a trout before the greater temperature. Higher temperature water, as you know contains less oxygen at saturation, but a trout will stray a few degrees from ideal to grab an easy snack.
A true "two-tiered" fishery will have prey items that also favor the cooler, now oxygenated water. I think some northern lakes almost act like a lake on top of a lake and have lake trout/cisco communities in the hypolimnion and muskellunge/sucker communities in the epilimnion. It sure gives fisherman extra options and also takes advantage of greater usable water volume.
