Dabuchi's feelings on this matter:

But what if I order a McDouble (two patties, one slice of cheese)?

"Dabuchi" stands for "double cheeseburger" which is kind of ambiguous. I kind of think two patties would be sufficient though.

Not sure what is on the JP menu, but menu-wise here if you order a Double Cheeseburger it's two patties and two slices of cheese, while the McDouble will get you the two patties and one slice of cheese, but it is slightly cheaper.

It might seem silly for the difference, but when you're paying 54 cents for an extra slice of cheese, for some that does add up over time.