While I haven't looked at the source, if it's anything like other prototype-based languages, they would either share the same prototype ("vtable"), literally adding the method to both at the same time, or it would be added as an extension to only one of them. (Being able to add a method to just one object OR all sharing a common prototype is a feature.)

Using prototypes, it would be one pointer. I don't see why that would be particularly space-inefficient.