Hi:
The short answer is: no, I have never tried it and no, I never would. ;-)
The long answer is:
I don't know from this what your regular routine consists of, and whether your body can handle running 30 minutes EVERY DAY, but I think you would be better off making that second workout a weight training session, rather than more cardio. and here are the reasons:
1) variety. Running is very hard on the joints and your body does not need a high impact, high intensity Cathe tape later on that day after this. It's too much impact. And injury could be right around the corner.
2) cross train. Even if you really want to run everyday because running is a fabulous calorie burner, I would want to mix up the cardio activities a little to prevent burn out, prevent injury and boredom. Your running can be better and way more powerful if you do it only 4 or 5 times per week and substitute other cardio activities of different intensities on the other days (kickbox is one idea and swimming would be my other suggestion, because it strengthens muscle around the knee that running cannot and which can lead to injury and a break from running if left undeveloped)
3) muscle up! Weight loss will be promoted more, not by adding a second cardio session, but by pumping up the muscle. Lean muscle tissue burns way more calories to maintain itself that other tissue, so if you weight train you will speed that weight loss much faster. You will also be able to develop areas of the body that running cannot target, including muscles of the lower body, and a more powerful body leads to more powerful running workouts.
Please believe me that in my 37th year of life, after listening to Aquajock, I commited to weight training and the results have been fabulous: I am trimmer, much more compact, I have better shape, a more balanced look between the upper and lower body, more energy even when resting, more strength, and much more power to channel into my explosive cardio workouts. I seriously believe a commitment to weight training will shed those pounds for you and take you way beyond this, in fact. It will transform your physique!
The solution to a healthier body is not more cardio, you already do enough.
CLare