Disclosure: I’m a private pilot an an ex-USAF avionics mechanic (C-5s). I understand how to trade energy in flight, and which systems can fail on large jet aircraft.
Losing both engines would be rare, but possible.
If it were to occur, the Unreliable Airspeed procedure wouldn’t be valid, as it calls for pitch & thrust combinations.
Based on aircraft weight, there's an ideal speed to maintain unpowered lift, and that speed is predicated on AoA flown down a glide slope optimized to convert altitude for distance; determined by the aircraft's flight path.
In a clean -16, 6o AoA is flown on a -11o to -17o glide slope. With AoA at 6o, you're in the ballpark of 210 Kts (all things equal, by dropping the gear and the airspeed will decrease to 200Kts).
Flying whatever pitch necessary to set artificial horizon to whatever pitch for whatever optimum glide slope, and fly whatever AoA will set speed needed per procedure mandated for unpowered flight in the FCOM, QRH or whaterver manuals / checklists they consult.
If L/D is 15:1, this works out to a glideslope of 3.8o. Too high AoA and glide slope decreases but airspeed will also decrease (eventually leading to stall onset), too low AoA and glideslope increases along with airspeed (at expense of height, distance and time for engine relight procedure).
I'm guessing at any altitude and weight there's a guestimate of the stall speed. Pilot would probably fly in and out of the stall onset region. Through use of a stop watch, an approximation of average velocity could be made. +/- 20Kts either side of that required for unpowered glide should be gutenuff for engine relight procedures.