reef ember
Through some means (probably a spectral card), allow a joker to become flippable.
When flipped, the joker initially just shows an empty back, which can be filled by buying another joker.
This basically could serve as an intermediate step between normal and negative jokers, allowing to add another one, but having to disable one of existing jokers to use it (and be able to toggle back and forth).
Downsides: Makes joker management clunky (can't reshuffle jokers as much, etc)
winter lark