Well, that exceeds my knowledge, but my guess is that it'd be very tricky to accomplish.

You have one cable, a given frequency may go in one direction or in another. Both sides have to agree on what it's being used for. You'd need a communication channel between the ISP side and the client side to constantly negotiate, and that negotiation would take some time, so such a mechanism would have some latency to it, with possibly weird effects on things like online games. You could get weird behaviors where some particular pattern of traffic would result in the connection readjusting itself just wrong on a regular basis and result in hordes of angry gamers.

I think it's reasonable to guess that ISPs targeting consumers have no interest in monitoring and troubleshooting such a thing when they can just set a fixed split and be done with it (and ask the customer to upgrade to a bigger plan if it's not good enough), and ISPs targeting enterprises have no need for it.

Edit: And there's the issue of how you sell such a thing. It's a system that readjusts itself automatically based on some arcane magic and may work differently from one day to another. How do you make any reliable promises about it?