(2) already happens, even if it's not automatic. The train that leaves Lisbon gets divided into two when it arrives in Medina del Campo; one part goes to Madrid, the other to Hendaye (France), where it connects up to the TGV.
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.
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.