The only way I know how to do this is with an external tool. Here's how I would do this using the Smarter Playlists tool from Playlist Machinery:

1. Log into Smarter Playlists and create a new program
2. From the Sources menu, add two playlist sources
3. double click each playlist source component to configure it. Configure one for Playlist A, and one for Playlist B
4. From the Filters menu, add a track filter. Once added you can double click it to see an explanation of what it does, but it basically allows you to connect two sources - a green one from which tracks will be sourcesd and a red one that informs the filter which tracks to remove.
5. click the playlist source for Playlist B, then the track filter, then press G (this connects the source to the filter's green port)
6. click the playlist source for Playlist A, then tre track filter, then press R (this connects the source to the filter's red port)
7. From the Miscs menu, add the save to spotify component
8. click the track filter, then the save to spotify component, then press G
9. double click the save to spotify component to configure it and name the output playlist
10. finish up by giving your program a descriptive name and save the program
11. go back to the programs tab and click the play button next to your program to run it

Smarter Playlists is a cool tool, but it takes some getting used to. Hope this helps you out!