Fine for the lowland sleeper but not really faesible for the highland sleeper because:
You would need an 8 car EMU, two 4 car EMUs and and a pair of single unit EMU railcars to form each service, each with a pair of cabs eating into passenger capacity. On top of that rather than the handful of spare carriages in use at the moment, you would then need at least one spare EMU of each length.
If you were going down this route, the better solution would be to split the Highland Sleeper into two services, a 9 car unit to Inverness and a pair of 6 car units splitting to form Fort Bill and Aberdeen services.
Maybe in 40 years time