Do I have to train upper and lower abs separately to get the best results?

No, you do not have to train the upper and lower abs separately to get the best results from your program. While it is fairly commonplace to refer to the abs as separate upper and lower portions, the reality is that it is virtually impossible to isolate the upper and lower regions of the abs.  Certainly there are exercises that emphasize different parts of the abs, but the entire abdominal region has to work together in order to produce movment or stabilize against movement. Therefore, the best results will be achieved through incorporating a variety of ab exercises into your routine, with each one providing a slightly different challenge and stimulus to the abdominal muscles.