I control a Sylvan Library and play with the top card of my library revealed (e. g. due to Courser of Kruphix). In my draw step, Sylvan Library tells me to “choose two cards in your hand drawn this turn. For each of those cards, pay 4 life or put the card on top of your library.”

Do I put the cards back one by one, so that the first one is revealed before the second one comes on top of it?

Nope. Both cards are put back at the same time, not one at a time (similar to Brainstorm). Your opponents will not know the second card down.

Nathan Long
Wizards.Com Boards NetRep