How does a math degree help in electrical engineering graduate studies?

Yes, it would help if you study Applied Mathematics.

However, how much it would help depends on your intended field of study. If you plan to go into Computational Electromagnetics or Digital Signal Processing or similiar fields, then it's great. If not, I advise you not to invest more than one semester in studying Math.

To give you a perspective: I graduated with a BS (GPA:3.5) and MS (GPA:3.5) in EE only. I took the subject Math GRE and scored in the 30% percentile.

I just acquired a bachelor of electrical engineering. In Brazil, engineering programs are 5 years by default. Next month I start my graduate studies and, from my experience, I think engineering programs should be more math-intensive (we do not have a mathematical analysis course, for example).

Let me explain why: I want to do research in computational electromagnetics and transients in transmission lines. I began studying more advanced topics in signal processing and numerical methods. I realised that I lacked the understanding of some mathematical concepts that appear in those topics (convergence, Lebesgue and Hilbert spaces). I also realised I did not have enough intelectual maturity because the mindset we learn in engineering is "do that to solve this problem", and not "show that this result is true and serves your purpose", which I think is more important; specially if you are going to publish a thesis and solve a problem no one ever did.

And as a side note: Claude Shannon, known for founding digital circuit design theory in 1937, did a double degree in electrical engineering and mathematics if I remember right.

To address your questions

How would a double degree in math and electrical engineering help me in electrical engineering graduate school?

Studying proof-writing mathematics gives you intelectual maturity.

Will I have any disadvantage if I just enter graduate school with a straight bachelor of electrical engineering?

I'd say yes IF you are going to a more math-intense area like signal processing. Because you will have to learn topics that are not present in engineering courses like convergence and Hilbert Spaces (at least in Brazil).

When it comes to graduate admission, does the graduate committee look more favorably to someone who has done a double degree in electrical engineering and math than someone with a straight electrical engineering degree?

I do not know. If I were from a graduate committee I would look more favourably to a student with a double degree in engineering and math.

If you enter electrical graduate engineering school with just a straight electrical engineering degree, would then learning the required math be doable?

Yes. I think that most graduate engineering schools assume their students have an engineer-only degree.