In the TV show, they would be unstoppable.
If Joker and Black Mask Appeared and Teamed Up In Young Justice
I honestly believe that nobody can seriously team-up with the Joker. He has his own agenda and would be a threat to everyone. That's why you see The Joker double-cross Black Mask in "Under The Red Hood".
But, yeah, I think it would be a great episode. :)
This edit will also create new pages on Comic Vine for:Beware, you are proposing to add brand new pages to the wiki along with your edits. Make sure this is what you intended. This will likely increase the time it takes for your changes to go live.