What you're trying to do is not a simple as it may first appear.
I see DVDFab's merge mode and DVDShrink have been suggested above but I don't think either of those will be able to directly reconstruct the menus from the split discs, as these are effectively re-authored discs.
One method would be to use the merged copy produced by DVDFab then follow this guide for adding the menus back in.
Alternatively you could process the discs in a DVD authoring package.
I don't think either method is particularly straightforward though.
TBH I think the simplest option would be to get the original disc professionally repaired at your local video store where they have machines that can polish out the scratches, but you can have a go at the above methods if you prefer the challenge.