I'm personally of the belief that we won't get a double reveal. Thus far pass 2 has cut the waiting time from reveal to release a lot, to where we only have to wait a few weeks or less for the character to become playable after their reveal. I kind of like this approach personally, since it keeps the hype flowing when you actually get to play the character.
With that said, I've been thinking about how a double reveal would go. I personally think Nintendo is going to have a lot on their plate this E3. There's BOTW2, that rumored Donkey Kong game, that rumored 2D Metroid game, and probably some other stuff that hasn't been leaked. I'm personally predicting a new Kirby game as well. Point is, they'll have a lot to show even without Smash. With that in mind it could be hard to fit in 2 Smash reveals in one presentation without going above the usual length Nintendo does with their E3 directs. So I wonder if it would be an interesting idea to show Master Chief (Were he to get in) at Microsoft's presentation, and another character in Nintendo's presentation? They didn't do that with Banjo, but it it also isn't something unprecedented, Mario + Rabbids was actually first revealed during Ubisoft's presentation, so Nintendo isn't always against these things.
To clarify, I absolutely don't think this is happening, but I thought it would be an interesting thought to share to spice up the constant discussion of whether we'll get a double reveal or not.