It could perhaps be argued to make sense to have a Liverpool-Glasgow portion on every Manchester-Edinburgh at most? Not sure there's that much demand though, and the portion working did have a habit of causing a lot of delay when that arrangement was in place with the 158s.
The only issue with that would be that you'd have two splitting points ?
If you just combined with some of the Manc-Glasgow services you'd only be joining/splitting at Preston which has plenty of capacity.
I think it is a route that has the potential to grow - much like Manchester - Scotland.