To create a dual KF8/mobi7 you need only define specific CSS styles for each and use @media to define when to use them. You shouldn't use different text for each format, just redefine the styles for each. See explanation below. So the textual content is not much bigger for a dual formatted file compared to a single formatted file. It would be a minute difference in size. However, if the book contained media such as audio and video, that could add a substantial amount to the file size. I have no idea if Amazon filters out the KF8 media portion when sending to a non-KF8 capable device, but I hope note. I can imagine that two different files might not whispersync correctly. Plus if you sync via WiFi then Amazon would not really save much by sending a smaller file. They would only save if the file was sent via 3G.

Example explanation:

Define a standard paragraph style for each format but keep the style name the same, for example p.myStandardParagraph. Use @media within the CSS to define p.myStandardParagraph as using black text for mobi7 and blue text for KF8. When the book is displayed on a non-KF8 device the CSS will format the text for p.myStandardParagraph in black and when the book is displayed on a KF8 device the CSS will format the text in blue.