Something I tried hard to do when I brought both Becky and Toni into the comic as twins was to NOT have them be identically repainted Megamen with boobs. I made subtle design changes from one to another. At first Becky only wore skirts and Toni only wore pants. Then I worked hard on personalities. Becky is such an otaku she dresses up as a Japanese school girl as her normal outfit and Toni wears a sensible ensemble. They have undergone such evolutions that yes they are based off of the same character model, but I have to remind once in a while myself that they are identical twin sisters.
By the way: NEVER call them "Bell Twins." They hate that.