The phrase "concentrate on both" is correct and usable in written English.
It is often used to emphasize that something should be done for two different objects or people. For example: "You should concentrate on both your studies and hobbies to lead a balanced life."
Dozens of collegians who thrive as both pitchers and hitters are forced to choose one or the other as professionals, the theory being that they can't possibly concentrate on both.
I guess my fear when I started at the Hairpin is that I wanted to freelance, but writing every day made me worry that I wasn't going to be able to concentrate on both.
"I cannot concentrate on both.
I wouldn't be able to concentrate on both jobs at once".
I used to run along the line that is painted on a track and I would concentrate on both feet hitting that line".
"I'm a person who cares about the present and the future," she said, adding that the first exhibition will concentrate on both the heroic skyscrapers of the 1920's and the new stars now rising in Asia.
Vietnam and the fight -- concentrating on both could really affect your mind.
It says it wants to become a more innovative consumer-products company, concentrating on both food and "personal care" (shampoos and so on).
The most critical stresses on the pipeline are concentrated on both sides of the landslide border and in the central part of the landslide.
The study concentrates on both software control strategies for the converters, highlighting the performance of the generator for the export of power to the grid.
Circulating fluidized-bed (CFB) technology has been successfully applied to the roasting of refractory gold ores and concentrates on both pilot plants (200, 350 and 700mm furnace inner diameters) and commercial CFB roasting plants.
