I've been trying to get the two discs on one. I have tried a couple of guides on AD but still have a resulting "oversized" combination after copying and compiling in Shrink.
I'm only interested in the main movie and dont want to cut out any credits or segments etc.
Like david66 said, you will greatly lose quality. If you want to have the movie on one disc, I suggest using DL media. It'll be much more expensive, but at least you'll get much better compression.
I crunched the special editions of the LOTR movies onto a single disc for each film using rebuilder and the quality was fantastic. Each film was 15GB or more, and the result was far and away above anything that Shrink could do. Then again it's a totally different process, and each has it's own strengths.
There should be no problem with your movie, you can even keep the menus and whatnot if you wish.