Why does that need a single thrust lever? The airplane can automatically pull back the individual lever for the failed engine, if they want to design it to do so. My plane does that for an unlocked thrust reverser, and it doesn't even have autothrottles.
The earlier linked article also talks about them integrating thrust more tightly into the control law, and automatic upset recoveries. But it doesn't say why either of those need a single thrust lever.
edit: OK, I can see one case: a descent where both levers are at idle. Then the single lever could prevent a misidentification and shutdown of the wrong engine. (second edit: changed "would" to "could." The computer could display "LEFT engine failed, shut down the LEFT engine, note: that is the same side of the airplane as your wristwatch and the side the guy with 4 stripes is sitting on" and someone could still get it wrong, unless they go one step further and change to a single shutoff button
)