Hi Capt Clarret,
You posed a hypothesis that can't occur and ALL pilots need to know it.
Under airline systems of maintenance, the same critical system on two engines may not be maintained concurrently.
This means that you can't change a fuel control unit on two engines at once.
The reason for this is that it eliminates the possibility of duplication of errors, as where an error is made the separation between the original task on system 1 and the duplication of that task on subsequent systems, will highlight the original error.
Airlines normally schedule AD's for multiple systems, even when the systems are non-essential or multiply redundant, across multiple shifts because of this.
Your original hypothesis that it is unlikely for two engines to fail is therefore correct, unless the cause of the failure is fuel, which is the only thing common to both engines.