When I was last controlling, the scenario you describe was basically correct. I certainly used it myself, but a/c A had to actually report leaving FL130 before a/c B could be cleared to leave FL140. Your post reads as though you are using Mode C to obtain a/c A's level as FL126. Don't know how modern technology has progressed, but in my day using Mode C in the congested area of a hold was fraught with danger due to garbling etc.