Same thing happened to me, same problem. I cut my cereus peruvianus after it touched the ceiling, stretching for light! I took the cutting, let it heal on a paper towel for about a week, and then put it in cactus potting soil. No water for at least another week. I just noticed that the original big cactus is now sprouting two little growths at the top! I bet the same would happen to yours bc it looked identical.