Hahah
In our case, bikini is how we call it in Catalonia because it was "released" when the nuclear bomb tests over there. Mixto is the Madrid version, and it makes sense because it's a sandwich with two ingredients, like the former word we used for schools with boys and girls. Then you also have to remember that we make a difference between sandwiches and bocadillos, the latter being made with French bread (or the copy of it), or basically, bread with a had crust.