Tragic accident and sadly two young military aviators lost.
Having flown approximately 150 hours on the T-6 during my pilot training, I can report that the aircraft exhibits very good characteristics for a trainer. Firstly it has a very comfortable cockpit layout, and more important, it is has very nice and forgiving handling qualities. It does not flip you upside down unless you mishandle it grossly, and it will let you know way in advance when you approach the edge of the envelope. It will talk to you with increased buffet and even a stick shaker, when you approach a stall, and if you intend to spin, you have to provoke it with crossed controls during entry and throughout the incipient spin. If you center up the controls and unload a bit during the incipiant spin, it will recover instantly. Regarding the spin in the T-6, I don't remember it as being flat-ish at all.
The Mk16 Light seats are very capable, and I am not familiar with any escape system failures with this type of seat in this aircraft type. Pull any handle, front or backseat, and the sequencer will eject the rear seat first, and the front seat in quick succession, regardless if the canopy det-cord works or not.
I wonder if they were either incapacitated, if the seat failed somehow, or if they misjudged the altitude needed for recovery, and therefore did not initiate the ejection. The report will show...