At or above FL270 set airspeed 275knots. Below FL270, set airspeed to 300knots.
I think those speeds are to help with engine restart and not to do with the best glide speed. At that stage of the checklist it is assumed that at least one engine will restart. Funny enough the last two options on the loss of thrust on both engines are either one or both engines restart. It mentions nothing about both engines failing to restart
