You are never going to get it for the current machines is my guess. Technically it should work with 2HD screens rather than 1 UHD screen, but I don't know.
It works just fine on any other OS for 2xHD screens, even Windows on MacBooks, so it's a software limitations.
That's an extremely stupid limitation. Every other OS does support this, and if Apple are so afraid of degraded quality due to lack of bandwidth, they can just put a warning when you do that.