You read the tweet wrong. It sounds like it works exactly as it works on Xbox 360 today (and maybe PS3, not sure, didn't buy Digital Games on PSN)
Let's say Person X and Person Y buy 2 Playstation 4s, okay? Now, Person X and Person Y want to buy Pokerman, but they want only buy one copy, but play it together at once, so they can have a Poker battle.
Person X buys Pokerman on his Playstation 4 (Playstation X). Pokerman is now licensed to Person X's account AND console. ANYONE can play Pokerman on Person X's Playstation 4. So now, Person Y can play Pokerman without paying, no restrictions, as long as he plays on X's console.
This is what they do, and what YOSP tweeted.
Person Y plays Pokerman on X's Console, no problems!
Now, Person X plays Pokerman on Y's Console. Y's console doesn't have the license to play Pokerman, but X. X can play Pokerman on any console, so X just downloads and plays Pokerman through Y's console.
So, exactly how it works now on the Xbox 360. This isn't an account being signed twice with no problems. It makes more sense if you read the full Twitter conversation.