There really is no reason to add champagne yeast, and it can cause a problem if the initial yeast had a stuck ferment.
Champagne yeast will not ferment any of the sugars left behind by the ale yeast, as a general rule, wine/champagne yeasts will not ferment complex sugars.
The reason some recipes call for adding champagne yeast is if the recipe is a high gravity beer and there is a chance the primary yeast can reach it's alcohol limit, or get too tired to carb when bottling.
In reality, most of the time the primary yeast is still viable unless the alcohol is getting above 10%.
If you really want to use it to help carb in the bottle, rehydrate it and add it to the bottling bucket rather than secondary.