If you reveal two Sphinxes of Foresight from your opening hand, you'll scry 3 twice; you won't scry 6. Any cards you put on top of your library the first time you scry 3 will be part of the second time you scry 3.

A player's "opening hand" is the hand of cards the player has after all players have taken mulligans. If players have any cards in hand that allow actions to be taken with them from a player's opening hand, the starting player takes all such actions first in any order, followed by each other player in turn order. Then the first turn begins.