I think that best holding speed is usually quoted as 1.1Vmd (for reasons of speed stability as explained above) whereas best driftdown speed would in theory be exactly Vmd. If we assume a Vmd of 200kts, the best hold speed would be 220kts which sounds reasonable for a medium-weight airliner. The best drift-down speed at high altitude would also be around 220KIAS which, due to compressibility, would give an EAS of about 200kts (Vmd).
Also, remember that the drag characteristics will vary between all-engines, wings level (zero sideslip) and engine-out, wings level (small angle of sideslip). The drag of the windmillling engine will also have an effect. Therefore the two configurations will probably have a different Vmd, even at low altitude.