A garden atonement seems odd for a couple of reasons. One, Paul always points to the cross where this event happened; never does he give any indication that the atonement was divided over two locations. In addition, the Bible only refers to the garden twice—never once in association with the atonement. Two, if Christ actually atoned for all of the sins of mankind in the garden, what would be left for Him to atone for on the cross?
Bill McKeever and Eric Johnson, Mormonism 101, pg. 158-159