All things being equal, it takes a lot more energy to slice through the air at Mach 2 than at Mach 0.85. But all things are not equal: Concorde flew about twice as high as regular subsonic planes. At that altitude, the air is about half as dense, so it takes less energy to move it out of the way.
In the end what matters is the lift to drag ratio and the speed ratio. In cruise mode, a regular commercial jet has a L/D ratio of about 17, and Concorde of 7 [1], i.e. about 2.4 lower. So each minute it spends cruising, Concorde will burn 2.4 times more fuel than a subsonic jet of equal mass. But Concorde flies faster, so it spends less time to cover the same distance. How much less? About 2/0.85 = 2.35. In other words, to cover the same distance Concorde burns about the same amount of fuel as a subsonic jet, while in cruise mode. Concorde’s fuel economy was horrible at takeoff though due to a triple whammy: L/D ratio of only 4, need to use the (very inefficient) afterburners, and the tyranny of the rocket equation.
But if someone can solve these issues, there is nothing that prevents a supersonic to be as fuel efficient as a subsonic jet.
[1] https://en.wikipedia.org/wiki/Lift-to-drag_ratio#Supersonic/...