You need to push the common code ...DOWN into a common assembly.
This is sometimes called a business layer....or comparable.
And then have both websites reference the same dll.
........
A website should be considered a presentation layer....and almost anything
you can push down a level will help you in the end.
You're fortunate in that with 2 presentation layers...you actually can
visualize the WHY you should push it down a little better than those who
use a single layer, with a single presentation layer.
Here is a sample of a layered application.
http://sholliday.spaces.live.com/Blog/cns!A68482B9628A842A!139.entry