It seems to me that "теперь ещё" and "пока ещё" have very similar meaning. But I am not sure whether they can always be replaced each other. For example, does it make any difference if "пока ещё" is replaced by "теперь ещё" in the following second sentence?
Уже не трудно читать русский текст. Но пока ещё трудно говорить правильно.