Choosing between letters of recommendation

I am in the process of preparing an application for a grad school/PhD program in Economics. The program I want to apply for requires two letters of recommendation so I was wondering which professors to approach to maximize my chances. There are basically two major research fields I am interested in and I could get a letters from a professor in each of the fields. However, there is also professor at a higher ranked university I attended a course at who could give me another letter of recommendation in one of the fields.

My question is whether it is better to have two letters from professors in a single research field rather than from each of the fields?