You ever heard the "pancake a day" analogy* for the workload in med school? A dual degree is the same thing, but with blueberry toppings added. Theoretically it's a few more calories you have to eat, but it's at least something different and not near as bad as the pancakes themselves.
There were certainly a few times I had to say no to a night out to work on MPH stuff, but I still feel like I have plenty of free time.
Pretty easy to get involved in public health research- it's just that many more faculty you have access to vs a MD-only student. Many students will turn their MPH practicum into a research project as well.
*
The Pancake Analogy