Thanks for posting in the Spotify Community; I think that this is a great idea as well!
There is currently a Live Idea in the Ideas Exchange of the Spotify Community about being able to shuffle 2+ playlists together, here; however, this Idea would likely necessitate saving the other users’ playlist to Your Library, such that both playlists could be selected from one account’s library in order to shuffle them together.
There is also another Live Idea that is similar to your suggestion, regarding generating a “Friendship Playlist” between two Spotify users, which would contain all the songs from all playlists saved by each of the two users.
If either of these Ideas are something that you’re interested in, I would recommend adding your +VOTE to them, as well as a comment for any suggestions that you may have regarding these Ideas.
However, if you’d like to see a feature specifically for shuffling two playlists created by two different users, without one user having to first follow the other user’s playlist, then I would recommend creating a new Idea for this in the Ideas Exchange, here; as well as first reading the guidelines for submitting Ideas to Spotify.
The more votes an Idea gets, the likelier it is for Spotify to implement it; and you can read more about how feedback on Ideas reaches Spotify here.
In the meantime, on the desktop app you can try creating a new, empty playlist and then using Edit -> Select All to select all the tracks from one of your playlists, before dragging them into the new playlist to add them to it; and then repeat this process with the other user's playlist in the case that you have it saved to Your Library or it is a public playlist on their Spotify profile. The new playlist that you made should then have all the tracks from each users' playlists, which you can then play on shuffle.
I hope this helps, and let us know if you have any other questions.
Have a good day!
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