Basically,this trick look really professional when performed.It will look as if you teleported the cross from one card to the other.To get an idea of what its all about,watch the video.If you still don't understand, proceed to the next step.You can perform it in the next 1 minute!
Mark one of the identical card.Make sure you mark on one side of the card,either left or right.Mark on both side of the card(back and front) and make sure its on the same side.(this will help in the performance,it will seems like it teleported through the card)
Get the full deck on your hand,place the marked card on top,followed by the other identical card.So you are now ready to perform.
Do a double lift and show that it has a marking.Then put the card back to the deck.Do some action as to show the audience that you are going to teleport just the cross, to the bottom card(rubbing,clapping)
Now,do a single lift and you will see the cross on the back of the second card.
Once again see the video,you will get a good idea from there.Its better to visualise from the video.