Reason I still feel a double reveal is that every E3 has had more than one Smash reveal. What makes this any different?
A lot. They have a hard two characters left. Revealing it now ends the entire core thing people care the most about. How they play is not going to keep the same hype energy going throughout the latter half of the year. It ends it way earlier than necessary, and they want to milk this as long as possible.
There's zero evidence we could've had a double reveal last E3 if it existed(the event id's suggested Steve/Alex was meant for there, but that's it. Min Min, considering she was delayed, was probably meant to appear before it. I feel like they already were going to do everything the same last year even if E3 went on).
Now if we had, say, an Echo Pass coming, sure. But we know there's absolutely no more characters(Sakurai hammering it home over multiple articles makes it clear it isn't an NDA case, since he'd only have to say it once and ignore the question otherwise. Or he's just ignore the question). Blowing it all at once doesn't do Nintendo any favors, and they chose to have it revealed within 2 years, with 6 full character packs. There's a huge amount of the year left, so just showing one is enough to get the train rolling. They'd have nothing to fill up the rest of the year, with zero new modes coming after all. Mode updates won't cut it either, since they're minor changes. The due date isn't just a "just in case". They still want to keep it close, so time is taken to balance things out. Characters actually do take six months to make for unique, so having a full year for three characters gives them lots of time to get it done right. And even Steve/Alex still had issues.
It doesn't mean it can't happen, but there's nothing all that convincing it will. The only reason it made sense for Hero and B&K to be together is they were basically having 6 characters released in one year. They had no choice but to double it up to keep it on schedule. Byleth was literally the end of the year too. I expect one now and one in September/November to finish it off.