There are tremendous time savings to be had between Aberdeen and Edinburgh, by calling at Dundee only
I've done this in a HST before, set failed and so ran ECS a few hours later to Edinburgh, and it managed to cut the journey time by about 30 minutes
Currently the NXEC Aberdeen - London Kings service typically calls at :
Peterborough, York, Newcastle (6 minutes standing time)
Doncaster - York - Darlington - Newcastle - Berwick (4 or 6minutes standing time)
Add to that the time for station stopping, I reckon there is a minimum 20 minutes time saving between Edinburgh and London Kings Cross, so the total might be as much as 50 minutes, which is quite near to what Platinum Trains had in their Press Release a few months ago, a journey time of just over 6 hours
As for two units, that is just impractical
However, if they are reintroducing the 05:55 Aberdeen - London Kings Cross this service can easily turnaround and operate a service around 13:00 to 14:00, but that is at almost the same time as a NXEC service
Furthermore, ICEC dumped this service many years ago as it pretty much ran empty to Dundee, due to no connections from the north
Equally, an early morning London Kings Cross - Aberdeen service, with a late evening Aberdeen - London Kings Cross are not likely to be well used