In that case, "B" did NOT have possession, A AND B had possession, and the ball belongs to A (you know that). And yes, that would be a safety.
But "B" never possessed the ball in the opponent's end zone, "A AND B" did. You can't parse the possession of B out of "joint possession".
If A and B are swimming, it follows that B is swimming. The 'and' is unnecessary. It's TRUE, but WITHOUT the 'and', it's ALSO true. I don't understand your logic.
You choose not to interpret the fundamental the way it is written.
I get what the rule is 'intending' to mean, but it doesn't actually say that. This is what's hard being an official and understanding the rules. I can read the rulebook and understand what it's 'trying' to say, even though it doesn't actually 'say' that.
Now, I can read it the way you are, which is technically incorrect, and be fine. However, many times a rule interpreter comes up with an interpretation that is zany, and other officials go, 'ummm.. yeah... that's what the rulebook says, we have to enforce it that way!' For example, the old rulings that the defense can score on a try (even though nothing in the rulebook said they could), or someone who is out-of-bounds but jumps up in the air and bats the ball back in bounds is legal. Crap like that.
Why isn't the 'fundamental' interpreted the same way until there is a rule clarification, but other crap is interpreted wrongly, and fully supported by many officials?