How does a dual degree (Computer Science/Maths) compare to a degree in a single area?
Your choice depends on your purpose and your competitors. You should ask yourself: who am I competing with?

If you want to pursue a Ph.D. degree, and if the subject you want to study contains a lot of math, such that a standard syllabus of a M.Sc. is insufficient, then you should go for it.

If you don't want to work in academy, then this depends on the company's choice of employers.

But remember, it is not about taking a lot of courses. As far as I experienced, professors pick their Ph.D. students by their i) research skills ii) background knowledge about the subject. And those are listed by priority.

Assume that, there are two CS students, Alice and Bob. Both are seeking a Ph.D. position in area of Game Theory.
Game Theory is closely related to Economics. But also, Multi-Agent Systems is main subject that relates Game Theory to Computer Science.

Alice chose to enroll a double-program which is a hybrid of Economics and Computer Science. Whereas Bob enrolled a standard M.Sc. Computer Science program, and took more Computer Science related courses.

They both graduated at the same time, with the highest possible graduation grades.

Scenario I

One of them wants to work at a company, and the other wants to join the research team of Professor Charlie. The result is, they are not competitors.

Scenario II

Both want to join the research team of Professor Charlie. There are two vacancies. The result is, they are not competitors (assuming that they are the only ones who want to join).

Scenario III

This time, there is only one vacancy. Now, you are Professor Charlie. Who would you pick? The right answer to this question is: what is my research team doing? Yes, it is indeed a question, but a necessary one in your situation. If professor Charlie's team is working on theories from field of Economics, then he will pick Alice. However, if his team is working on pure computer science and Multi-Agents, then he will pick Bob.

However, above all these, if there is a third student, Daniel, whose grade is not so high but published two top-conference papers and submitted one of his papers to a top journal, all in the area of Game Theory, it is most likely that Professor Charlie will pick him.

All in all, 


  If you don't know where you are going, any road will get you there. -- Lewis Carroll