I know. I have experienced it. But I have experienced it as an very time consuming process. I don't see why this can't be done in an automated way, taking minutes. Not hours.
Provided that both trains arrive on time.
And doing in in flight is much harder (and not legally possible), but if you would want to join/separate in the outskirts to get one into town and the other on the bypass, just the stopping and reaccelerating will cost some minutes.
In the UK this is known as "portion working" and is a routine part of some routes' timetables.
The slight difficulty doesn't come from the separation, which is easy - open the automatic coupling, drive away the front bit and then drive away the rear bit after the front has got clear - but the joining. With an intensive timetable, waiting for a (possibly slightly late) other portion can leave a train missing its booked slot into London, with knock-on delays.