I guess it's not possible within the app?
I think I'll try sending two playlists to my Mac ,erase them on the iPad then combine them on my Mac and share back to the iPad the combined playlist.
(For those wanting to know a way to do this on these devices at the moment, choose the shortest playlist, tap the first song, add to the long playlist (tap and use the appropriate button), swipe to next song, add to that playlist again, etc. Then change the playlist name if required.)
I had two playlists , Rock 1 and Rock 2. They had a lot of common songs and each list was over 100 songs.
Sharing them both to the Mac program, combining them and sharing back to IOS was actually a fairly quick process.