Repeating what many have said already, in a slightly different way. An aicraft that flies at high altitudes and high Machs will have a "Mach Transition" or "crossover" altitude. Varies a bit with the day's weather (and aircraft type), but it is the altitude where Vmo (in IAS) equals Mmo (in Mach number). For most aircraft today that is in the region of FL260, although I have seen a few mentions, for particular airframes, of FL300. (and for Concorde, ~51300 feet, I believe - 440 knots all the way up to 51300, and Mach 2.02 higher than that - and coming down, 350 knots until 34-35000 feet, where the aircraft dropped below Mach 1).
Above that altitude, you fly with Mach as the speed reference, and by definition, if you do not exceed Mmo, you will never exceed Vmo.
Below that altitude, you fly with IAS as the speed reference/limit, and by definition, if you do not exceed Vmo, you will never exceed Mmo.
My impression, not having tried them all, is that today's FMCs handle the transition automatically - the aircraft "knows" its own limits, and will use whichever speed is lower (strictest limit). With an optional manual IAS/M selection button.