I reckon they will probably lay 1 tracks worth of ballast and move 1 track in with gantry etc then deliver the ballast and everything else for the second track from the adjacent first track. The first will take while but the second would be very quick, this also allows vehicle access along the second track bed for while longer.
Still can under stand why something like the Balfour new track train isn't being used especially mid week during daylight hours!
I suspect someone may have got bit excited when suggesting September and might now be regretting it. They really need all the concrete finished this week.