Here is a freebie. Use an impression device. Peek a spectator's choice of card written on a pad of paper. Have the spec lock that pad of paper in an envelope and keep it under lock and guard (well have them hold onto it for you). Do a regular routine with cards, not asking for their written card. In fact, do an ACR with a different card chosen from a different spectator. But during the ACR give that other spectator a few glances and make eye contact, and say simple things like, "I know what you are thinking"... continue the ACR, look over again to the first person and now say "I feel you, don't worry" and tap your breast pocket. When you finish the ACR with the other spectator, maybe in CTM or Braue pop up, go onto one last phase. Do Card to Wallet to Sealed envelope, and this time... there are two cards inside the sealed envelope. First pull out the ACR card for the 2nd spectator, and lastly, after having the original spectator open their envelope to reveal what card they wrote down, pull out the very same card from the same envelope as the other card.
No signatures, but one card is bent, and another card was written down and predicted. No signatures, no need. Adding signatures might just be a bit over the top actually.
What sells this is not the fact that you make a prediction. It is the subtle pauses you take during the ACR to give a bit of mysterious attention to the first spectator. You are of course still fully invested in making the ACR for the second spectator as much about them as possible, but you act as if your brain gets interrupted by the thoughts of the first person. You make your small gestures or remarks, shake it off, and pretend you are back in focus with the ACR.
These emotionally selling points are what make the effect big, not just the fact that you did something with two people.