Thanks so far ... extra info, one would qualify for a 26-30 railcard, the other would not.
Will splits do anything?
The West Coast London-Brum-Scotland (Glasgow/Edinburgh alternate hours) service is mostly Pendolinos, leading to some good advance fares from Birmingham to Edinburgh, especially in first class. That will be the split worth trying.
As has been suggested, a Two Together railcard will pay for itself on this trip.
Obviously the time of year will not help, but we got Brum-Edinburgh return for £100* each first class just before Christmas last year.
* net of railcard discount.
No fares are up on EastCoast yet for that date - which may be good or bad!
WCML▸ is the way to go - it's a long way in a Voyager.