sh_ and ch_ compose the same words

[Torsten]

Let me ask you Torsten, in your view, could you answer :

from the medieval manuscript viewpoint, for a random hypothetical invented alphabet

If we would use the letter c similar to the letter k, but have a slight preference for one of them,
would you then more likely see if they are 

a) different
b) they behave similar

c) neither

I would expect that both letters are interchangeable and that in one context letter 'c' is used and in another context letter 'k'. It would for instance surprise me to see multiple instances of 'color' and 'colour' in the same paragraph and I would find it stunning to read 'color' and 'colour' along with 'kolor' and 'kolour'.