Sorry people, but a double exposure is something we used to be able to do with film, expose the same frame twice, hence its name. What is being described is two entirely seperate individual different exposures blended together to make a single image - this is called exposure blending, three images & it becomes an HDR. If mutliple exposures of the same exposure are blended, then the result is a composite.
Sorry to be pedantic, but to some of us it's confusing when the wrong terms are used.
Smite away if it makes you feel better!