Hi there @Houselover,
thanks for reaching out to the community !
In order to merge two playlists (on the same account) you'll have to get access to either a PC or MAC.
Then, use the traditional Ctrl+A, Ctrl+C, Ctrl+P to select all tracks, copy them and paste them in the desired playlist.
Please bear in mind that these actions aren't available on any mobile device.
Hope this helps.
Let me know if you have any further questions 🙂