They are free turbine engines so they are never coupled.
The L/D ration only works with both engines secure and your at the best speed.
The only thing that can be inferred is they had substantially less effective power than they required for flight.
The reason for that lack of effective power be it high drag or low power on the live engine we will have to wait for the report.
I am pretty sure though its high drag due to low speed and an unsecured failed engine or one that is producing some power but not above the zero thrust level.