Both Vx & Vy (single engine or all operating) are determined by the drag vs thrust curves for the number of engines operating.
The power required curve changes with increasing altitude - as does the power available. This changes the relationship between these curves.
Vx occurs where there is maximum excess thrust, Vy where there is maximum excess power.
The speeds at which these two different excesss happen gradually closes as altitude is gained until they become equal ie Vx increases while Vy decreases. When they're equal then the a/c has reached its absolute ceiling. At this point there is no excess power OR thrust available to support further climb.
If they didn't become equal then it would lead to the nonsensical prospect of flying at the absolute ceiling while maintaining either Vx or Vy, then when climb performance reduces to zero, changing speed to the other of Vy or VX & then continuing to climb - in which case you couldn't have been at the absolute ceiling because there is still some amount of climb gradient AND rate available.