You can't currently do it with the Ripper module, although this feature has recently been requested and may be added in the future.
If you use Merge mode in DVD Copy, you can make it into a single DVD folder or ISO, but it will still be two titles on the DVD and will convert as two distinct files in Ripper.
If you convert each part to a single file in Ripper, you can probably find a third party file joiner with google that will put them together. Be sure to use the same settings for the conversions.