The rules have definitions on control that reasonably cover this scenario, I think. Here is how I would paraphrase a ruling based upon the rules, but keep in mind that the official Q&A is a better place to answer this question.
As soon as the ball touches the 2nd bot’s internal hopper, the 2nd robot has control of it. If the ball is in flight, however brief that flight is, then no robot has control of it. While a ball is contained/controlled by the first bot it is not controlled by the 2nd bot, even if the first bot is hovering over the 2nd bot. Note that this doesn’t cover a ball touching the ground, which gets into a different meaning of ‘control’ w.r.t. moving robots and incidental contact.
To be honest, if a ref has to study a moving robot to determine whether it has control of 5 or 6 balls, we can’t fault the ref for throwing a foul in the event that the timing is “just right” to not have actual control over 6. So for a real match, tune the timing to keep it obvious!