'Masked Singer' Wild Card Is Two Celebs With Public Drama Between Them

Get ready for some history to be made on "The Masked Singer" ... because there's going to be two celebs in the same costume, and we're told they have quite a history between them.

For the first time on the FOX competition show, there will be a double-faced costume with two people inside the same mask ... and they're set to debut on 'Masked Singer' as the beachball.

Production sources tell TMZ ... the two celebs hiding out underneath the beachball haven't always seen eye to eye, so it sounds like there could be some drama on tap.

And, get this ... our sources say the celebs making up the first-ever wild card duet have combined for a pretty up-and-down past few years ... which could make things super interesting.

The big question, aside from their true identity, will there be another public blowup between them?!?