I think it is better for you to use a [Spotify music downloader][1] to locally download songs from Spotify for offline use in your slideshows. Such a tool supports batch conversion...
Sorry. I have no experience with WDC. Maybe you can visit its official website to check out its how-to guide. Alternatively, use other software if you don't mind. Share a useful post...
Do you mean to join two video files into one? If this is the case, try a [video joiner tool][1]. If you have not your favorite one, you might as well try Joyoshare Video Joiner...
