Are these slideshows you created in iDVD from still photos? If so then there is no way except to put all of the photos into one slideshow and make a playlist from the songs in both individual slideshows and add it to the combined slideshow.
If the slideshows are QT movie files from iPhoto or iMovie then you can create an iDVD slideshow and add the two movie files to it as you would still photos.
Do not use any transition between files and select any slide duration except manual.
Also I suggest you follow this workflow:
Once you have the project as you want it save it as a disk image via the File ➙ Save as Disk Image menu option. This will separate the encoding process from the burn process.
To check the encoding mount the disk image and launch DVD Player and play it. If it plays OK with DVD Player the encoding was good.
Then burn to disk with Disk Utility or Toast at the slowest speed available (2x-4x) to assure the best burn quality. Always use top quality media: Verbatim, Maxell or Taiyo Yuden DVD-R are the most recommended in these forums.