No way to store preferences. As each person reads they will have to change the font size to the one they want. It basically 2/3 clicks to change the font and takes a few seconds (font button, pick font size and push the joystick toggle).
The only way I can think of to save separate bookmarks would be to strip the DRM and save a second copy of the book with a slightly altered titled (you can do this with Calibre's metadata function). That way they each have their own version of the book with their own bookmarks and notes.
They can also read the books on an iPhone/iPod Touch and with the Kindle for PC app. If they turn syncing off and use the search function or notate the location of where they stopped reading, they can continue to read on other devices.
Edit: They should really just buy another Kindle.
