Can a notary back date an acknowledgement if the person who needs the notary also back dates?

A notary does not sign to acknowledge a document. A notary is a witness of the person who signed the document. He signs to say that the person who's signature appears on the document is, in fact, the person who signed it. (This is why a notary will ask to see your drivers license or other photo ID - to prove that you are who you say you are.)

A notary would back date a document because the person signing the document already signed and dated it before it was handed to the notary. In this case, the notary would not have seen the person sign the document, so it is illegal for the notary to both sign and back date.