The two cards you draft are drafted one at a time, in case one of the cards cares about when it was drafted or what the next card you draft is.

You may use the ability of more than one Cogwork Librarian while drafting from one booster pack. For example you can draft three cards from one booster pack and put two Cogwork Librarians into that booster pack.

Cogwork Librarian essentially lets you draft two cards from a booster pack in exchange for putting Cogwork Librarian back into that booster pack.