May/Pac was literally a once in a lifetime event where fans had to make time at a given time or miss out on it. With DD S2, fans who will binge it starting midnight are the same ones who will most likely watch BvS at 7pm or so. For everyone else, setting aside 3 hours at some point during the weekend is not hard, a lot of binge watchers don't exactly watch more than 1-2 episodes at a time anyway.
Also, we have literally no idea how many people watch DD, Netflix never releases viewership numbers whereas May/Pac was watched by over 50M people in the US at literally the same exact time. Those are 2 completely different things. I would wager that DD S2 might affect BvS somewhere between jack-squat and literally nothing. It misses the "must watch it live at this very instant" buzz.
I was on the Box office forums on AOU OW, before May/Pac AOU was tracking to a 62-65M Saturday which then became a 57M Saturday, so even with the large viewership, it affected the OW by 8M at the very max i.e. less than 5%. Also, the lost viewers were regained on Sunday when AOU only fell 10% as opposed to a 30% fall which is normal.
The only good thing for both BvS and DD S2 is that they will both get a lot of free press for this in the media for some time.