Speaking as an electronics engineer with a bit of aerospace experience, I would first suspect the physical components between the thrust levers backwards to the place the two signals fan out to the engines. From what I've read, if the EEC is disconnected from the thrust lever the engine will soldier on at it's current thrust level. In that case, one plausible scenario would be that the thrust lever assembly became either functionally or physically separated from the EEC's. This might happen if the thrust lever resolvers shared a common power supply or the box connected to the thrust lever resolvers had common power or clock circuitry. In the physical realm, I'd look for a broken cable or connector that carried the signals to both engines.