Which yields more pokemon - two active lures at once, or two lures used consecutively on one stop?

Using two lures at the same time vs. two lures consecutively should actually yield the same number of pokemon. The only difference is that you might catch a few more if you use the lures consecutively, but that's only because you would have the app open for 30 more minutes.

If you're going to be using a lucky egg as well to get double XP from catches, definitely use two lures on nearby stops. Otherwise, it's really up to personal preference.