petite question technique

Sylvan Library
Color= Green Type= Enchantment Cost= 1G LG(U1)/4(R)/5(R)
Text (5th+errata): At the beginning of your draw step, you may draw two cards. If you do, 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. [Oracle 2000/02/01]

Spells and abilities are resolved one at a time, so if you use multiple Sylvan Libraries in one stack, each will resolve in sequence. You do not get to draw all the cards at once then put them all back at once. [WotC Rules Team 1994/09/15]
You always resolve your normal draw before this ability because the normal draw is placed on the stack after this one is. [D'Angelo 2001/12/16]
You can return zero, one, or two cards. [WotC Rules Team 1994/09/15]
If you only get one or fewer draws due to this effect (because the other draws were replaced), you still have to put back 2 cards if possible. Any cards drawn this turn are applicable to this. This makes the Aladdin's Lamp replacement ineffective at escaping having to put a card back, since the Lamp does put a drawn card into your hand. [D'Angelo 1997/05/21]
If you manage not to draw cards or draw less than 2 cards, you do not lose life by not putting the undrawn cards back. Thus, you can use Mangara's Tome to get a card or two instead of drawing and avoid having to put cards back. [Aahz 1996/10/21] Abundance can also be used to avoid draws and cause no loss of life. [D'Angelo 1998/10/15]
If you have drawn cards prior to your draw step, they can be ones chosen to be put back using this effect. [D'Angelo 1997/03/21] For example, a cantrip (see Rule E.3) draw or an instant used during upkeep.
If you are going to draw cards due to triggered abilities during your draw step, your regular draw resolves first, but you can choose to order the Sylvan Library before or after any other draws (such as from Howling Mine). For example, if you have two Howling Mines, you must do your regular draw first, but then you could do the Library (draw 2 and put 2 back), then your 2 other draws. Or you can do your 2 draws, then the Library (which lets you put back any 2 of the 5 cards you drew, including your regular draw). Or you can do some of the draws before and some after the library. [D'Angelo 2002/04/20]
This will count as 2 draws for anything that affects "drawn cards". [D'Angelo 1995/07/11]