Pure impossibility with two signed cards!
A spectator signs a freely chosen card. The performer also signs a random card.
You give the spectator your signed card and show them that you have their signed card in your hand.
Now you give them their card and take yours back. Somehow by magic, even though you clearly traded the cards, you show that nothing has happened -- you STILL have their card and they still have yours!
This is Bluff!