If you have 2 of this card on the field, their effects are cumulative.
If your opponent has no cards left in his/her Deck, he/she cannot declare an attack while this card is on your side of the field.
Netrep Rulings:
While “Gravekeeper’s Servant” is active, discarding a card from the top of the Deck becomes a cost to declare an attack.
If a player does not have sufficient cards remaining in his/her Deck to fulfill the requirements of “Gravekeeper’s Servant,” that player may not attack.
If the opponent has more than one “Gravekeeper’s Servant” active on his or her field, the player must discard 1 card for each “Gravekeeper’s Servant” when he or she declares an attack.
The card sent from the top of the deck to the Graveyard is sent even if the attack is negated if “Gravekeeper’s Servant” is active on the field.