Hello RAT 5,
We don't really accelerate but basically maintain the same Mach Number since we are cruising in average at M.78 on the A 320. And the speed target for an engine failure in cruise is .78/300Kt.
I asked Vilas his opinion about why it's 280kts for dual engine flame out and 300kt for single engine flame out in cruise and he said:
"Relight speeds are there to ensure required air density and N1. While the speed of 300kts in engine failure cruise is from mandatory requirement for twins on non ETOPS to have an alternate within one hour on one engine. That requirement is considered to have met at .78/300kts."
So I guess the 300kts is just to comply with regulation. So the theory I heard from one instructor that we used 280kt instead of 300kt during dual engine flame out to minimize the descent rate might be rubbish after all. Haha.