From Scott Jenson’s 2021 post about Tesla 3 interface, this is so clever (emphasis mine):

Edward Tufte has this visual rule that 1+1=3: With a single line on the screen, you have just that single object, but adding a second line does something interesting, it adds a third ‘object’ on the screen, the negative space between the two. All good visual designers deeply understand this effect.

In UX design we have a cognitive equivalent. If you have two buttons, there is a third ‘object’ created: the decision a user must make on which button to tap.