That's the Seattle solution.
But think of the infrastructure. You would have to have segregated approaches to the T5B platforms on the "inbound" side (T5B->T5A), and segregated exits from the T5A platforms.
And think of the capacity issues. Broadly speaking, the train has to take as many people from T5B to T5A as it takes from T5A to T5B. But if you take out one carriage for a sterile return, then you cram arriving passengers into half the train space. A wonderful way of generating complaints about the warm welcome to the UK.
And, as I say, all this would be for a mere handful of people.