In the first instance, would you not still end up with the DVT in the middle anyway? If the loco was on the London end southbound and you put the Edinburgh on the back of the train...
In the second instance, would that not result in both DVTs facing each other in the middle of the train?
Either way - how are you then going to turn the train at London for the next northbound trip?
No, No and the train doesn't need turning.
You haven't thought this through. Why on earth would the loco be on the London end. The whole point of using DVTs is that the loco can PUSH the train.
I see I'll need too spell it out move by move.
The required stock for this (Nbound and Sbound) is 3 locos, 4 portions of sleepers and 2 DVTs.
Start at Wembley. Formation South to North is DVT, Edin portion, Glas portion, loco.
Move down to Eus DVT first, goes up to Carstairs loco first.
At Carstairs, loco takes Glas(front) portion to Central, loco 2 drops on back portion, takes DVT and Edin portion to Edinburgh. Edin loco runs round at Waverley, takes portion back to Polmadie, runs up to Central light engine, brings Glas portion back to polmadie.
Next evening, loco 1 plus portion without DVT runs ecs to Waverley and runs round. Loco 2 plus Glas portion plus DVT runs up to central, loco leading, DVT trailing.
Both then work down to Carstairs. Loco off Glasgow portion detaches, leaving DVT plus Glas portion in the platform, goes to stabling point and waits to do the Northbound Edinburgh portion later on.
Edinburgh portion (no DVT) arrives, runs into the headshunt and drops back onto the Glasgow portion.
You now have a train in the platform consisting of (South to North) DVT, Glas portion, Edin portion, loco.
This runs down to Euston DVT first, back up to Wembley loco first. We are now back at the beginning again with the same formation, the same way round.(Glas and Edin portions are identical, so the fact that they've swopped positions is irrelevant)
At no point do you need extra ecs locos and at no point do you get a misplaced DVT.