I take your point (EM2) about the rebuilding work, and possibly the southern end problem.
However, the dwell time at St Pancras is quite long compared to other stations in that section. Having the ability to put two units in together could speed up getting the trains out of section, especially if the outside platforms could connect straight on to the ECML connection without having to run in front of a MML service.
I can see trains on the ECML blocking the entrance to Kings Cross if it is held to wait for a train standing in the existing platforms.
Incidently, does anybody know if the layout between Copenhagen and Gasworks is going to be upgraded. At present, there would be conflicting moves between south running trains going to St Pancras and north bound Kings Cross departures. If the exisiting four tracks were moved across one tunnel to the east (in to the spare tunnel) the 'free' tunnel could be used for direct access to St Pancras. There is plenty of space to have a grade seperated junction north of Copenhagen. Thoughts...