Seriously, don't think of doubles as splitting. Think of it as adding. Do your main run just the same as you normally would, and then add on another run (most put it in the morning) that is light and easy enough to not interfere with your primary run.
At first this is just going to seem like a trivial little thing, but soon you'll notice that even with that you're getting faster. Whatever it is that running twice per day does, it doesn't really need all that much of a stimulus to make it much more productive than a single run. Just make sure not to shortchange your primary training. In fact, there's usually not any reason to change that at all.
That's why the discussion of whether 6+4 is better or worse than 10 is pretty weird. The real question is about whether 10+2 or 10+3 or 10+5 works better for you. And also keep in mind that what works best for you now may not be what works best for you at time x in the future. Hopefully your training will keep changing your body for the better, and then different training will work optimally for you.