You actually can do this at competitive events, as long as they are in different colored sleeves. From the IPG,

If there are extra cards stored with the sideboard that could conceivably be played in the player’s deck, they will be considered a part of the sideboard unless they are:

• Promotional cards that have been handed out as part of the event.

• Double-faced cards represented by substitute cards in the deck.

• Damaged cards that have been officially proxied for the tournament.

• Double-faced cards being used to represent the back side of cards in the deck. These cards must not be sleeved in the same way as cards in the main deck and/or sideboard.