Is it okay to focus the PhD Research Proposal in more than one topic?
The function of the research proposal depends widely on the nature of the admissions process. If you are being directly hired by and supported by a professor's research group, then the proposal should be directed to the interests of the particular group. 

On the other hand, if the department handles admissions and assigns students to groups later on, then you do not need to focus your interests too narrowly. The big question is this: are your interests in the "hot" areas, or other areas within the department? If you're applying in the "hot" area, then you need to be more specific (since there's more competition); if you're doing less popular topics, then you can afford to be less specific.