Complex sentence: The interstage truss between the second and third stages twisted apart and the latter separated from the stack and at T +50 seconds, the cutoff command to the first stage was unblocked and the engines immediately shut down.
Split into simpler sentences: The interstage truss between the second and third stages twisted apart and the latter separated from the stack. Meanwhile, at T +50 seconds, the cutoff command to the first stage was unblocked and the engines immediately shut down.