Is it possible to lose two cards in one turn?

Yes, you can be knocked out instantly by an Assassinate action. This can happen in two ways: (1) you try to challenge the player claiming the Assassin, and lose the challenge; (2) you claim the Contessa to counter-act the Assassinate action, then you are challenged on this and lose the challenge. In either case, you lose 1 card for the lost challenge and then the other from the Assassinate action.
Related Rule(s)