As far as I know, the only time the Joker and the Royal Flush Gang have gotten together was early in INFINITE CRISIS where the Joker took the Gang out because he was annoyed at not being asked to join the Secret Society.
It sort of seems obvious to me that the playing card motif should have meant they’d team up at some point, though?
Crisis on Earth-Prime 
I’ve been trying to find them teaming up as well and can’t. For such an obvious fit, you’d think they would’ve worked together, well, FOR Joker not so much WITH him, as he’ll always want to be the boss in any team setting. I guess he sees them as pretenders, or stealing his gimmick or copying it but badly? Very interesting to see what should be an automatic pairing instead be a total adversarial relationship.
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There’s an episode of Super Powers Team (the last season of Superfriends fully rebranded for the Super Powers toyline) that features the Joker’s one appearance in the Superfriends series and it also stars the Royal Flush Gang.
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