So... what you want to say is... if you add two numbers right now, you get the correct result, and if you add them later, you get a fake result
Yes, double/multiple exposure means adding (or averaging) some image buffers, usually in RGB space. Probably it works in YUV too (didn't try).
Example (averaged 1800 movie frames in RGB):
http://dl.dropbox.com/u/4124919/long-exposure.jpgIt was done with a python script, from an 1-minute movie => it's totally fake.