Real world deicing can be done BOTH in one step OR in two steps, they are both equally valid and are both commonly used. We just chose to reproduce the single-step procedure, which makes more sense in a simulator, since it wouldn't make any difference if we just repeated the same animation with just a different color.
If animations lasted 1:1 with reality, they will be simply too boring and would get old very fast. That's why we don't load 300 baggages on a 747...