See Chapter 19 Halacha 5 for the full details of this subject! It is debated amongst the Poskim whether an after blessing for one food automatically covers another food that was eaten even if one had explicit intent to exclude it. The ramification would be regarding if one may continue eating the second food without repeating the before blessing over at, and whether a second after blessing is mandated even if he does not eat the Shiur Kezayis/Revius by his second eating if he did so before. Practically, in order to avoid the dispute, it is best to either:

1. Delay the after blessing until after finishing eating/drinking all foods.
2. Exempt all the foods in the after blessing, and then repeat a new before blessing for the food that one desires to continue eating.
3. One is to have in mind to exempt only one food, and then change areas from inside to outside or vice versa, and repeat a before blessing on the second food which he plans to continue eating.