Stilton
I don't have any definitive information but a U2 (T-bird) crashed in September 2016 in California the instructor was killed even when video shot at the time showed two ejections and two parachutes, which appeared to have opened normally at a good height (i.e. well above the ground).
I can only speculate that the rear seater may have struck the front seater's canopy (or his seat) and being killed as a result. There's a thread on PPruNe with the photos I refer to. I have not seen any accident report published but I am hardly going to be on a distribution list to receive one!
MB