This was always my biggest dilemma when I was inpatient and started on double breakfasts. In the end I went for toast and then cereal whilst inpatient, for practical reasons - we had 30 mins to eat breakfast, and the earlier it was finished the earlier we could start (and therefore finish) supervision, so having toast first meant not having to wait for it to be ready when breakfast time had already started.
Since being home I have switched, and tend to have cereal first and then toast. I think just because I prefer the toast usually, although actually I'm not sure this is the case anymore. Oh the dilemma!