I know the relationship calculator will display all relationships between two people, but I don't see in the documentation where you can set which one displays on an on-going basis. I know for a fact that this can't be done on the online trees, and as far as I can tell same with FTM trees. Both will show the "Closest" relationship (e.g. 12th GGAunt is fewer steps than 13th GGMother).

I agree with you it would be a nice feature but I don't think it can be done.

Ancestry doesn't monitor these forums so I would suggest you email the request to support@ancestry.com and maybe they'll add the feature in a future update. I'm doing the same, I think it's a good idea.