Yes, but you also have the extra weight of the second engine - and you have to transform that mechanical energy to electricity before the electric engine transf

Yes, but you also have the extra weight of the second engine - and you have to transform that mechanical energy to electricity before the electric engine transforms it back to mechanical energy again, which is lossy. So all in all I think it makes sense for long range/remote areas, but I rather would have a fuel cell as a range extension. (which has its own downsides of course)